Great job once again, WillLem!

The nice irony is that, due to the low resolution of Lemmings games in general, the diagonal line still looks a little like a staircase, indeed, because one can still recognise "steps" in it. I don't think that's a problem - it just goes to show that there is little difference between a ladder and a diagonal rope in practice. Only the Platformer doesn't have this feature, since he creates a straight line.
Now we have an analogous symptom to the "longer step / brick" (the "a little bit further forward" part you mentioned) that we had with the angular ladder, where two consecutive ladders won't form one straight diagonal line, but there will be a slight horizontal shift instead. I don't think this is a problem either, since the resulting "ceiling" is flat, so several ladders in a row are still Shimmier-friendly in one piece. This probably just has to do with the starting position of the ladder.
The other point you raised is related to that, I assume:
2) Lems can currently ladder through existing ladders, Builder bridges and other thin diagonal platforms. Do we want this? Personally, I think it's desirable, but I can see a case for it not being so:
I've noticed something similar with the Laserer, who will not cut through a Builder staircase if assigned on the last pixel before stepping through the staircase. The reason for that is that he actually holds the cannon above the staircase, so it makes sense he can't shoot through it. A similar behaviour can be exploited in NeoLemmix by having a Shimmier jump through a low-hanging Platformer bridge (the Reacher state takes him above the bridge, so that he then lands on top of it).
So I don't think this is a problem either. In fact, it can be useful to connect Builder staircases and ladders in a zig-zag pattern.

1) The Ladderer currently stops when the end of the ladder hits terrain; they should also stop when hitting one-way-fields/Blockers. Turning wouldn't work unfortunately (for many reasons), but stopping seems appropriate.
Ah, you mean that the terrain created by the Ladder can't be affected by Blockers and one-way fields? Makes sense, just like a Spearer's or Grenader's projectile can't be affected by them. One-way fields (which Blockers have on their hands) only affect lemmings themselves, at the end of the day.
However, what happens when you assign a Blocker in front of a Ladderer to turn the lemming himself around mid-performance? Here I'd expect the same behaviour as when turning a Laserer with a Blocker mid-shooting (I just saw a NeoLemmix level on which this is required

). Meaning, the Ladderer would create the lower part of the ladder in the opposite direction - similar to what happens when you clone a Ladderer.
Also, I just want to say I like the skill panel icon! 
It's just a little strange to me that the downward staircase in the icon looks like a Builder staircase, i.e., more "step-like", and at a flatter angle. I assume this might just be a preliminary version of the icon, and the final version will have the actual angle the skill uses?

If not, I don't have a problem with it either - it might just be a little more confusing for new players.