Every music here is from online game "TalesWeaver".
Xmas_01: We wish your Merry Christmas (https://youtu.be/DJ1qOW0xJC8)
Xmas_02: Where santa goes (https://youtu.be/DJ1qOW0xJC8?t=73)
Xmas_03: Feliz Navidad ~ Christmas Tales (https://youtu.be/EaQYDVG20mA)
Xmas_04: Have yourself a Merry Christmas ~ White LEAF (https://youtu.be/RU6VkioAv44)
Xmas_05: Brand new Spring is coming (https://youtu.be/xgHOl5RWFu0)
Xmas_06: Good Evening Narvik (https://youtu.be/Lctr21iN_u8)
Xmas_07: 열지 않았던 작은 상자(translated to "A small box that hasn't been opened") (https://youtu.be/7LqIIYyt46I)

- You need to use miner to the first obstacle and laserer to the second obstacle.
- The miner destroy some of the builder sacks and this is crucial to the solution.

Retro 1 Nice first level to the pack - not too difficult but not a pushover either. The worker lemming (or rotten egg) needing to take the top-route rather than following the same path the rest of the lemming would take was a good detail.
Retro 2 Nice semi-open ended puzzle around containing the crowd. I was convinced a save-all solution was possible here, and eventually found something that worked using some blocker trickery at the start.
Retro 3 Good early level, saving the climber using a basher was a nice detail though the timing was a little tight on my solution.
Retro 4 Nice builderless level! By bumping up the RR I was able to save a 2 miners which I imagine are required to do a bit more crowd control if you don't do this.
Retro 5 A nice challenge to try and save as many as possible here with an open skillset. I managed a save 29/30 in the end but I tried for a while to get a save-all by using the builder + digger + builder trick to turn a lemming around, but couldn't quite connect back to basher tunnel without running out of skills to get back up.
Retro 6 Nice puzzle with two distinct workers meeting up - not too hard but satisfying to solve
Retro 7 Very nice idea with re-using have all the blockers get freed to become blockers a 2nd time. The terrain hints to place the blockers was a nice idea to reduce execution complexity on this one.
Retro 8 I like these sort of levels where it is about efficently solving a seemingly simple looking terrain area. It wouldn't surprise me if there more than one solution to this given the skillset and terrain shape, but I settled on an approach using two different digger pits.
Retro 9 I really liked the multiple workers interacting on this one, and the timing with the icicle was very satisfying when it just worked out
Retro 10 Very clean solution, and very clever. The top-left hatch was probably the area I wanted to build bridges with the most, but the solution cleverly requires patience with this hatch in particular. I really liked that no timing was needed in the solution to this level - I did initially think the solution might involve 6px steps around the bottom hatches and some clever timing, but those solutions all ran out of builders.
Retro 11 Another very good puzzle. I found this a little easier than 10 but that's probably because I spotted how to handle the bridge area quite quickly fortunately. Not using a blocker to turn the miner, which is the initial temptation for this level, was a nice twist.
Retro 12 This is a beatiful solution and probably going onto my LOTY list. There is some very nice subtlety to where you need to place the skills on this for it to work out, and when they are placed correctly everything clicks into place in a very satisfying way. An early solution I had that didn't quite work (but thought it might have been the trick) was to put a builder ontop of the walkway after the exit to turn the lems around - I was quite pleased with spotting this as I thought I had to the key to level, but nope.

1 01 Last One Out is a Rotten Egg - Intended! You figured out that the hero lemming who prepares the path for the crowd should be the last lemming spawned! Otherwise all following lemmings will splat and you can't waste any builders in this level to break the fall.
1 02 White Christmas - My solution takes the top route but your solution is also fine
1 03 Snowy Wish - Intended! You can turn the climber around first and make him build twice. Then the timing is not needed.
1 04 Claustrophobia - I've received quite a few different solutions from multiple people. And I would say your is also a clever alternative solution! Check the replays pack above(main thread)! I included various solutions from people as well as my own one.
1 05 So Close, yet so Far - Saving 100% is possible! (there's the 100% replay in the replay pack! )
1 06 Christmas Crossing - Again 100% intended!
1 07 How to Escape an Avalanche - Acceptable! You didn't put one of the blocker at the correct position but the main spirit of the level is still there! The level is set up in a way that the blockers are freed in perfect timings so they can join the second part of the level in correct timings. The 4diggers+4blockers are for destroying the icicle terrains and you have to build spiral staircase to the exit with all 6builders and 3blockers.
1 08 Harvest of Screams - That's 100% intended! This level has several possible solutions and all of them require two different digger pits as you did!
1 09 Rendezvous at the Cavern - 100% intended!
1 10 And There Were Four Again! - The solution was made based on my solution to original dirt level "And then there were four..." And yes, I know I complitely overcomplicated the original level when I was a kid. I added some wall-traps from pillar tileset to make my old overcomplicated solution into the intended solution.
1 11 Roof of the World - Perfect! If you figure out that you have to separate 4 athletes and contain rest of the crowd with a blocker, the solution will comes to you naturally.
1 12 Forbiddiden Knowldege (Part 1) - Glad you like this level! This is also one of my personal favourite levels of this rank. Originally there was a sequel level named "Forbiddiden Knowldege (Part 2)" in the second rank but I ended up rejecting the level as the solution was quite hackish and requires some obscure trickery.

1 Frozen Fractals All Around - As you said the difficulty resets at the start of the rank. Not too difficult but again still a bit of puzzle here with using the slider.
2 Meanwhile in Namidaland - Nice decoration on this one! I ended up doing something a bit fiddly on this one and I'm sure there is a cleaner way to handle the crowd.
3 Xmas in Lemmingland - Using the walker to not need to complete the bridge to icicle was probably the trickies part of this one. Another nice puzzle using the new skills.
4 We Move Unseen - I liked the two paths on this one, and the helper markers in the floor to line up the laserer was a nice detail. This was a fun one to watch complete afterwards too.
5 Ice Climbing - The levels started to get a bit trickier here! There are some nice details in the solution here with how the 2nd climber is used & letting the first climber get back up.
6 Santa Training Center - A great single worker level. I really like the way the miner continuing is used twice - the first one is quite telegraphed but the second one is much more subtle. I saw in the intended solutions you even did the thing I did in mine where I checked both bridges worked before mining the left one.
7 CAUTION Thin Ice - This is quite a tricky one! The key for me on this one was realising where the jumpers had to be placed at the end and working backwards from there.
8 Last Shepherd of the Dead - Great use of the zombies on this one & the little hiding area is a fun idea.
9 Get Your Sled Ready - Nice level combining a couple of tricks together. The level shape very strongly hints at continuing a skill, but also using compression and the stoner + blocker effectively adds some nice extra complexity.
10 Sorry No Rooms Available Tonight - My favourite of the rank - The way the blocker lemmings are used on this level is really elegant.
11 This Level has no Time Limit - This is a fun change of pace level and pretty tricky to work out what actions you can spare on the main worker. In particular the details about holding back the crowd to allow the bridge time to complete & the oh-noer still blocking the zombie make the solution to this quite well hidden. Very nice puzzle.
12 Presents for You - A great level to finish off with thematically and in terms of puzzle - Initially I wanted to collect the basher pickup above the water, but that caused problems later on when trying to use the cloner effectively.
13 Thanks for Playing - Credits

I also checked the replays & liked the 100% solution for Retro 5!

2 01 Frozen Fractals All Around -  nice solution! As long as you don't let the whole crowd to take the bottom route(which I think is impossible) it's fine!
2 02 Meanwhile in Namidaland - Yeah it's more of an visual level than a clever puzzle level. Any solution is acceptable in this level.
2 03 Xmas in Lemmingland - 100% intended!
2 04 We Move Unseen -

Quotehelper markers in the floor to line up the laserer was a nice detail

Yes I tried to minimize execution difficulty. That's why there're markers in the floor in this level, 1 07 How to Escape an Avalanche and one level in Bonus rank.
2 05 Ice Climbing -

QuoteThere are some nice details in the solution here with how the 2nd climber is used & letting the first climber get back up.

Exactly! Especially if you don't see that you have to break the builder bridge with the backward miner, you'll have a hard time solving this level.
2 06 Santa Training Center - 100% intended!
2 07 CAUTION Thin Ice - Ha! that's actually nice logical deduction! I wanted the players to think that they have to save multiple jumpers and a basher for the last part but it seems like I failed successfully here. Good job on not falling into my trap!
2 08 Last Shepherd of the Dead - I made a cozy shelter for that shepherd lemming.
2 09 Get Your Sled Ready - This level was made using bottom-up design tactic. I first depicted the sled with snow tiles and developed the solution based on that.
2 10 Sorry No Rooms Available Tonight - Thanks for the compliment! It's important to realize that you can't use all 3 stoners at the bottom and you can afford only 2! And you also have to figure out that you have to send the first lemming to the bottom and made him a blocker and release him when the stoner bridge is finished!
2 11 This level has no Time Limit - I wanted to include at least one action-based level with many jumpers and here it is! You contained the crowd at different location to me but it's totally fine!
2 12 Presents for You - Good job 100% intended again! The most difficult part of this level in my opinion is figuring out when and where to use the cloner.

I was quite surprised that you've found intended solutions to every single level so far!
Bonus rank is mostly untested. So I wonder if the current trend continues...

1 Lemming Entanglement - This is an excellent level . I think this is probably the hardest of the 3 I've played so far. I ended up solving this one by thinking about the level in reverse and asking about if it could ever work if I: (1) built from right to left from any of the 3 junctions or (2) Use the teleporter to bring the crowd down. After ruling those out I was able to focus on the other possibility and spotted the trick with releasing blocker with the digger which unlocked the rest of the level.

2 No Matter Where You Came From - Another great difficult level, and the verticality makes a nice change. I was able to find the route down quite quickly but, the main bit of difficulty was optimising enough skills & working out how to create a filtering device for the swimmers & sliders similar to the one used at the top of the level. There probably is a cleaner way to do this than how I managed it, but hopefully I'm not too far off with this one.

3 Hold My Beer... - This one looked like it was going to be difficult from the preview screen, and it certainly was. I had quite a few ideas that either ended up a skill short or a lemming short. I included a replay with one of my closest failed solutions where I was missing one floater at the end. My solution probably brute forces a part of room 2 a bit to make sure all the lemmings came out the pit in the same way - I'm guessing there is a cleaner way to do this part by saving a miner elsewhere which I probably missed. Very nice puzzle