But what about the furniture in my room? I didn't buy any of it, and I doubt I'll take it with me when I move out, so... is it mine or theirs?
It's theirs, unless they say otherwise.

And my clothes and other personal items, I wear/use them, but I didn't purchase them.
They're yours.

I'm also wondering about gifts in general. If it didn't cost me anything in the first place, how do I determine replacement value?
This is where you should talk to an attorney.

Oh wait...

Any advice aside from "you should talk to a bankruptcy attorney" would be greatly appreciated.
Sometimes - MOST times - that's the only logical answer. Every BK case is different, and if you think your details can or should be handled by anonymous law generalists on an internet forum, you may well be setting yourself up for trouble.