Sounds about right except for the 'build high quality levels from the get-go' part because pretty much everyone is going to make a few levels that are hot steamy garbage when they're just starting out. Until they pick up on the quality that the community has established and gain some experience with level design themselves, most people are going to make a few stinkers. I feel like I am a good example of this; while Lemmings Squared does have some good levels, it has its share of terrible ones too. Having gained more experience and feedback, I feel like Casualemmings has a lot fewer levels that are objectively bad.

I still feel like I did something I shouldn't have with 'Final Impediment.'
I'm thinking maybe just have the water under the middle column in 'If at first you don't succeed.' I like the normal solution I use of sending an athlete up to build the way then mining through the third pillar and bombing through the stairs. It feels a little less janky than what I had to do here.

On another note, I finally solved 'No justice for the hero.' No idea why that one took me so long... Made me feel like a total moron the whole time.

These are actually new features getting implemented in the switch to the new NeoLemmix format, from what I understand. Release rate is getting a change where every distinct rate will be different. In the original games, only every other number was different (97 = 98, 95 = 96, and so on). In the new format, every release rate will be different. 99 will still be the same, the new 50 will act like the old 1, and every number lower than 50 will be even slower.
As for the ability to open the options menu and level select even while playing, I believe that is intentional as well. If you've played DOS Lemmings or similar you may be used to having the function keys like F3 be used as hotkeys to select skills... I can see how you might be annoyed when F3 is suddenly opening an options menu instead.

I also like that the platformer doesn't turn. Making it different from how it works in NeoLemmix allows for more puzzle ideas to come to fruition. For similar reasons if jumpers or shimmiers get introduced into NeoLemmix, I feel like they should be different from how they are in Lix (no tumbling mechanics, climbers can't cling to a wall if they jump into it, etc.)
I find marking Lemmings to be useful for when there's a Lemming in the middle of the crowd that I have to have do something by separating from the crowd, but I don't know exactly which Lemming it has to be to make everything time out properly. Marking the Lemming helps me to know which one I tried last. Still, that's a case that only pops up very rarely.

I've already made some changes to Power Core, but I'll wait to update this level pack until I get feedback on all of the new levels. I'd rather release a fix to all the broken parts at the same time.

Version 1.1 is out now!
Check the first post in the topic for the updated .nxp and music pack.
I said I would add 5 levels, but I ended up adding 15. Also fixed some backroutes, shuffled levels around, and more.

I wouldn't mind if the ceiling did become deadly... The only level I can think of that I've played recently where I used the ceiling was "Bridgeworks" from Sublems, where I built all the way up to the ceiling to form a wall up there, which I don't think was the intended solution anyway.

I also agree with the 'comb' case. Ending up walking on the lowest ledge seems to be the thing that makes the most sense there.
I'm guessing there's gonna have to be some messing around with stuff to figure out what looks and feels good. After all, it's not that often that a person codes everything just right on their first try anyway.