i just "finished" the enchanced crossbow schematic quest. for this, i needed to go to mzinchaleft. i did my best, cleared the dungeon and defeated the centurion, but i couldnt reach the chest. i go to the wikia to see how i THEN need to get to the chest, only to see you need some sort of item from another quest for it. this seems not to be working as intended, as i needed to use TCL consoel command to reach it without going on another quest.

Yeah, I think you have to almost complete the CoW to even get in there.

The chest with the schematic should be in the bandit part of the ruins, not in the automaton part of the ruins. Did you have your active quest set to the ancient knowledge quest? I think your problem is you went too far and now the game is showing you "the shortest way" to your objective which is through Blackreach and then back to the upper levels of Mzinchaleft.

When visiting Mzinchaleft, I would recommend talking to Mjoll first so you can get her sword while you are there. I would also recommend picking up the attunement sphere from Septimus as well so you can open the gate to Blackreach.