After killing the boss in a large room before the locked door, all I get is some garbage but not the Focusing Crystal. I've tried to load a save and enter Mzulft again - same result. So what should I do?

Follow the quest marker? It's on a Falmor you kill in the room prior to where you meet the guy that asks for the focusing crystal. The big dwarven Sentinel machine only guards the key that is required to reach the guy that requests you get the focusing crystal.

its on a falmer in the big room prior

Guys, I KNOW where it SHOULD be (Falmer-boss in a big room), but it just ISN'T there ^^ Anyway, something went wrong with a quest so I just had to use console and get the item.