Do we know exactly what trap doors do to monsters?

+max monster level?
+monster difficulty?

Is it worth going down trap doors at lvl 55+?

Makes monsters higher level, items can drop above your level bracket but general consensus is the difficulty is not worth the reward.

If you have the gear... 2 or 3 should be okay... but otherwise just remake game (in reference to solo'ing)

Monster level goes up in accordance the difficulty as well. Think it is around 3 levels higher than your current level when you enter, so if you can gain like 3 levels killing effectively you should catch up... just don't hope for too many consecutive TD's after each other.

Source incomprehensible at times

Normally I would agree, but that's where I found my only notable drop of last year. 185ee lvl 45 earth

Even traps (real ones) are escapable on occasion, so that may be considered a good thing

+1-5 lvls per trapdoor over normal driver lvl's monster spawns.
This can result in higher level drops of items and maps. It can also allow lower level players to drive meaningful monster levels for higher level party members.

Since the rewrite, TDs affect maps as well, but TD levels cannot cause monster levels to exceed the map lvl cap.

But there’s no trapdoors in maps.

Correct. You cannot find a trapdoor in a map. If you want to map with one or more trapdoors, you must open the map in a trapdoor game.

Interesting to know. So were you informed of this or did you do extensive tests? (which in most cases you would... )