Merge Sort -- I'm happy you like it.
There was some discussion on where to rank it, as I was afraid it might be too obvious while other players (I think Simon?) got stuck. I'm glad there's also some middle ground.
Derailed Level - On the other hand this one took a lot of attempts. For a while I was using the cube to block the other lix instead of just digging. I also didn't consider having any lix other than a climber building the platform, trying to go from right to left using the curved motion lines of the train. I feel a little cheated that the derailed train is not at all part of the solution, at least, not part of mine.
The train was a late addition -- Simon was unhappy about both levels looking the same and people confusing them, and he felt they needed something to put them apart and make them more memorable. So Simon added the train, which I think was a good idea and achieves this quite well; but that also introduces a red herring.
This Is Too Much Stepping Stones -- I have never seen this solution before! I'm sure, somewhat ironically, Insane Steve would be delighted to see this solution.
This level was inspired by an alternative solution/backroute to the original Lemmings 1 level Stepping Stones. Steve turned the backroute into the main route of this new level...and you backrouted it!
Cornerstone -- Thanks for the praise!
I was concerned the level might be a bit fiddly, but maybe it's alright after all.
Repeating Patterns -- This one turned out well by total coincidence. My usual modus operandi for making levels is having a trick in mind (e.g. from a failed solution attempt for some other level), and then building a level around it. I made the fractal and slapped a generic "10 of some skills" skill set onto it, and it turned out that with bashers this makes for a decent puzzle.
Cool to hear you're a maths teacher! Quite a few people in this forum have a background in maths or at least CS, and I've also done a few maths education related things (running maths camps, resource creation, tutoring...). I feel in some way there are quite some parallels in designing levels for puzzle games and designing a sequence of exercises for a maths topic, with exercises (ideally) building on top of each other and following logical progression to deepen the understanding of the concepts (which in puzzle games equates to the mechanics and solution strategies).
Duality -- I believe that this is a backroute.
There is a way of only using 3 bridges at the bottom, and this changes quite a lot about the solution overall.
Gomen Ne Sunao Ja Nakute - Not sure if my method is the only one? Those bridges only just fit again, but when I tried the route through 4 pillars then 2 above I found myself a bridge short
You can make the solution idea in spoilers work, you just need to figure out the crowd management. I guess your solution would be considered a backroute, but Proxima should confirm.
Rumble to the Bottom
The reference solution uses the same skill set as your solution, with only one placement slightly different.
Two For His Heels -- You haven't requested hints yet, so I don't want to give a full fledged hint sequence yet (the way Forestidia does it seems great btw), but maybe just one vague starter hint, in case you want to look at it:
Quite a few levels in this rank have been about crowd management where not just the skill placement, but also the timing was of importance.