You can bash through the ceiling in stepping stones (Mayhem 17 I believe) and you can dig and bash through the floor under the columns in Taxing 1.

Maybe Eric can add in the ability to scroll through the levels that you've already completed on the level select screen using the arrow keys? The SNES and Genesis versions let you do this. Then maybe he could put an option in a .cfg file to have all the levels unlocked by default.

I've solved all the levels. Hundreds of times. I shouldn't have to put in a long and annoying password to get to the level I want to play.

Which DOS ONML Tame level is, without use of possible not-yet-known glitches, solvable, but not 100%-able with only one type of skills?

I don't think any of them are? I just played through all the Tame levels and I was able to get 100% with only one type of skill on all of them except for level 5 and level 20, and I don't see how you could complete either of those levels with only one type of skill. (I guess you can save a few lemmings on level 5 using only diggers but not nearly enough to pass the level.) Of the remaining levels, only level 7 and 14 required any thought as to how I could complete them 100% with only one type of skill (both used builders).

Edit: Scratch that, I just saved 56% using only miners. So I guess level 5 is the answer.
Question: Which ONML Tame levels can be completely using exactly one skill?

I always do them with RR set to 73, so that you only need to make every third lemming dig (the other two will survive the fall).

I have confirmed 100% solution for the Genesis version of Mayhem 18... highlight to read the solution

Have the first lemming dig down. When there are two lemmings walking in the pit, make one of them bash right, then when you get the chance make another walker bash left. If you did it right then the lemming that bashed left will be the only one that makes it to the area below; all the other lemmings that walk in the left bashers path will turn around before he finishes until the digger prevents any more lemmings from escaping to the left. Make the right basher build to stop, then make the digger build to stop as soon as he is deep enough to trap the lemmings. The lemming that bashed left should make it to the area below, where he should build once onto the metal thing, then use one builder to hit the metal wall on the right and two more builders to get higher up against the wall on the left, then use the last basher to get onto the hill where the trap is. Build two bridges over the trap to the thing on the left, then build once more to get to the exit, then use your final builder to free the lemmings to the left

Added 100% solution for Taxing 6 - Compression Method 1

I don't think I'm going to bother trying to improve my 49-second solution for Just a Minute Part 2, too much trial and error fiddling with the release rates. There are 19 seconds on the clock when the first lemming finishes bashing, and it takes about two seconds to get from that point to the exit. If you can make it so that the last lemmings to turn around take 1.5-2 seconds to reach the wall on the left (plus another 1.5-2 seconds to come back), then the maximum time left to complete the level should be 13-14 seconds. If somebody wants to try this using my method then go ahead.