1) With this logic walking lemmings should not turn around.
Well, usually they don't.
A builder bumping his head on the ceiling will leave a gap there, because he can't finish his skill, and walkers will just slip through it, no matter how tiny it is, and keep moving forward.
The only case where walkers would turn around on the ceiling is when a fencer clears a path right to the top of the level and you have a lemming walking up that slope. Then again, now that the walker is also a skill of its own, you could say that skill of walking gets interrupted once the lemming hits the ceiling. And a skill being interrupted this way usually leads to the lemming turning around, as it also happens with a miner or basher hitting steel.
2) Shimmiers usually won't stop when they hit a ceiling when jumping upwards, because they actually want to reach a ceiling. So the rule "Any skill should get interrupted when hitting the (terrain) ceiling" will no longer hold. So why should it hold with respect of the ceiling of the whole level?
You're right that a skill wanting to interact with the ceiling is the unique and new part about this, which creates an exception to the rule. Bringing back the example of the swimmer, it would mean "all skills get cancelled once a Lemming's feet touch water", and the swimmer is the only skill which can still be assigned when the lemming is already in the water (stoners, bombers, and cloners can only be assigned in water once the lemming has been made a swimmer).
So here comes the philosophical question again of whether the ceiling is a steel wall. Most skills can't interact with steel. So it could be worth contemplating whether the shimmier shouldn't be able to shimmy along steel surfaces. It would make sense from a flavour perspective, at least - one a plain steel surface, he'd have nothing to grab onto
. Mechanically however, I think of the shimmier as a horizontal climber, and the climber can of course hold on to steel.
I'm certainly not advising for a change of the climber mechanics here, that would be insane
. Rather, we could create a difference between shimmier and climber here, similarly to the builder vs. platformer-discussion which popped up recently. Here also the L2-feature of the platformer gaining height was removed on purpose.
So perhaps a shimmier shouldn't interact with steel, even though it is able to in Lemmings 2? Like with the platformer, we'd weaken the skill in comparison to L2, but thereby would also make it differ more from an existing one.
L2 blurred the lines anyway by additionally introducing the diagonally climbing rock climber, plus the option of assigning the shimmier to a lemming hanging from the ceiling. If I understood your plans correctly, this isn't supposed to be possible in NeoLemmix. So instead, it would have to be done with a piece of terrain next to the wall, making the climber a glider so he lands on top of it, and jumps at the ceiling as part of the shimmier skill from there. Or, as I like to do it in many of my levels, one would have to use two climbers, making one of them a stoner with the other one landing on top of it, so he could start shimmying from there.
How is the ceiling regarded in the current stable version, anyway? Does it count as "terrain", i.e. do lemmings do terrain checks with it the same way they used to do with the level sides in 1.43?