I have completed the pack. Here are my replays. In addition, I have my complete LP of the pack up on my Youtube. The link is here:
Kaywhyn's LP of Doomsday Lemmings
Note: These replays of mine were made in an extremely old version of the pack, which is labeled v2.03 in my LP, in v12.9.4 of the NL player. When upgrading to v12.10.0, 4 of the replays break, and the replay refresher tool does not fix replays as old as v12.0, as apparently I played the pack that's much older than v12.7. So, if you want to view these replays, make sure you're using v12.9.4 of the NL player or older.
Regarding the pack, this was a really good pack. Just like you said, it starts off very easy, but then it starts becoming difficult at the very end of the first rank. So, I am in complete agreement that this pack is not for beginners. Here, the difficulty picks up with the penultimate level of Outbreak, Outbreak 6 -
Cursed Afflication. However, I thought it was very easy, but that's probably because I backrouted it. Outbreak 7 -
Undead Dipper, was a tricky one to figure out. Somewhat hard, but not a very hard level. I think I ended up overcomplicating my solution compared to yours.
Once you hit Plague, the second rank, the levels start getting difficult. Right out the gate, Plague 1 -
Cycle of Undeath, starts off difficult. While not very hard due to the very lenient skillset, it's difficult to figure out how to avoid getting infected, especially the left entrance. The rest of the ranking wasn't bad at all. Plague 2 -
All in the Rhythmn didn't take me as long as I thought it would. It almost fails had the trap triggered again before each group is able to get past the trigger, but it's a very slow trap to re-trigger, and so none of the regular lemmings die to it. Plague 4 -
Not Such a Stronghold... was the hardest level of the rank for me. While still somewhat open-ended due to the skillset, it's still difficult. The biggest reason why I struggled with it was due to trying to maximize the amount of lemmings to save. Once I realized the huge amount of lemmings we're allowed to lose in the level, I had an easier time, although the level's still no cakewalk even after realizing that fact. My favorite levels in this rank were both Plague 5 -
Care Package and Plague 6 -
Terror Cove. The former's an excellent puzzle where the solution really comes together and was very satisfying to figure out, while the latter's a really great puzzle in figuring out when to release the worker lemming, and the builder placement is crucial in allowing the space cushion to allow the crowd to safely be released and exit before becoming infected. It's just a very nice puzzle that I absolutely enjoyed. Well done, namida!

Finally, Plague 7 -
Braaaaains, while somewhat easy, was still a fine level to close out the rank. My solution might be a backroute due to having a builder leftover, but at the same time I could had used the builder instead and so have a platformer leftover instead.
The entire Apocalypse rank was just one very huge struggle. Because of this, my LP ended up spanning way more videos than I thought it would. Originally, I thought this pack was going to be a quick and easy solve from start to finish, but that was far from being the case. The levels that took me the longest in this rank were Apocalypse 5 -
Garden of Trauma and Apocalypse 7 -
Release the Horde!, as evidenced by my LP with how these levels took multiple videos.
The first level of the final rank, Apocalypse 1 -
Research Center, was where the difficulty really hit. This was a difficult puzzle to figure out, particularly how to dispose of the zombie horde and making sure you don't run out of platformers. Apocalypse 2 -
Infection Bypass took me much longer than I thought it would and hence it's a surprisingly very difficult level. The skillset is extremely limited, yet that's not why the level is hard. Instead, it's timing the release and making sure to place your skills carefully so that no more than 3 get infected if you elect to use all 5 blockers. The most difficult area is the level right above the exit floor level due to there being 10 zombies on that ground level. Apocalypse 3 -
The Longest Way Home was the easiest level of the rank and took me the shortest to figure out, but that's probably because I backrouted the level. Apocalypse 4 -
A Strange Facility was also another surprisingly difficult level to figure out. The execution was quite frustrating to do. It's probably yet another case where I overcomplicated the solution.
I'm not sure why it took me a very long time to find the solution to Apocalypse 5 -
Garden of Trauma. As seen in my LP, I ended up doing so many wrong approaches in multiple videos before I finally hit upon the correct way. Here, I think the reason why the level is hard is due to how the solution is extremely well-hidden. In particular, it took me a very long time to figure out what the purpose of the bomber and the floater were for, as well as how I kept using the bomber incorrectly. If I'm not mistaken, this is one of the levels that Akseli couldn't figure out. It was also the case for me, but I eventually figured out the solution on my own after a lot of struggling.
Apocalypse 6 -
One Lonely Lemming is probably a level where I really overcomplicated the solution. Instead of getting rid of the first zombie the lemming starts in, I simply kept moving along to avoid infection. In addition, despite being a 2-of-everything level, the skillset also makes the level hard. As you can see in my LP, I got taken by completely surprise upon not hearing the exit unlock after pressing the two visible buttons in the level. Here, I didn't turn on CPM just because I rather experience playing the level from a first-timer playing the level and therefore didn't want to take the easy way out and cheat. I managed to correctly deduce where the hidden button was. Overall, I still thought this was a really nice level, even if it was somewhat frustrating due to the way I possibly might had overcomplicated the solution.
Finally, Apocalypse 7 -
Release the Horde! deserves a very special mention.
I seem to be the very first person to officially find a nuke solution to the level! The only reason I thought the nuke was required was due to how I thought there wasn't enough skills to complete the level normally. This came about while I was trying to solve the level on my own. After the third video of my LP on this level, my plan was to simply record this level as a post-commentary video after I had solved it. After several attempts where it failed, I decided to record with the level unsolved. The level was still a no solve when I started recording, and as can be seen in the final video of my LP, it still took me a while to get it to work. I thought there was no chance that it would even work after I realized that the timer was just starting way too early in the cramped group, but I eventually got it to work. Also, I believe this is the second level that Akseli couldn't figure out. It was the same with me, but I did figure it out on my own.
Being the curious person that I am, as can also be seen in the finale of the LP, I watched your intended solution to the level. The reason why I struggled greatly with this level and hence why I kept thinking I was always at least a skill short is simple: I never thought about releasing the horde the way you and others did. No wonder I struggled so much with the final level of the pack.
I was quite surprised to see that the nuke isn't required at all and I therefore overcomplicated the solution to the max on this level. However, I perhaps should be praised for finding a nuke solution to the level. This means that we can now add another level from a custom pack where a nuke solution exists. For me, this would be the 4th or 5th level I know that a nuke solution exists, although this would be the very first one I played where it's not 100% required. Let me be clear that although the nuke isn't required, it still took a very long time to spot. As a matter of fact, it nearly fails anyway.
I still thought this was a really good pack that I still enjoyed despite struggling so much throughout nearly the entire Apocalypse rank. Definitely agree with you that the pack is not for beginners. Except for josh, I think I'm the 2nd person to solve all of the latest version of this pack? So, we now have 2 sets of complete replays for this pack, in addition to yours. Oh, I should let you know that 2 of your replays that you posted in the OP,
Outbreak 5 and
Apocalypse 6, don't work. They both fail due to a misplaced skill assignment. To be fair, the replays are for v12.7, and I played the pack in v12.9.4. Thus, I ran a mass replay check on your set in v12.9.4 and both Outbreak 5 and Apocalypse 6 fail. All your other replays work. Just thought I let you know. Of course, I still managed to figure out all the levels on my own
As for me regarding zombies, I don't mind them, although I can definitely understand why others might not like them so much due to how the puzzles can be fiddly and frustrating when they're in levels. I still managed to LP the entire pack without really raging at all. As a Lemmings player, it takes a lot in order to get me to rage at the levels. I guess I just have a really high level of patience compared to most players when solving. After all, I didn't get through all of United without a lot of time, dedication, and extreme patience with the pack. Same with your Lemmings Plus Alpha pack.
Enjoy my entire set of replays and my LP to your pack!
