just finished Angel Beats 10

I'm confused.
1.

Spoiler

So what happened to Yui? Did she disappear?

2. Do they go back to their old life if they fulfill what they want in that world, or what?

3.

Spoiler

If Yui dissappeared, why didn't Hinata?

 

1.

Spoiler

Yes Yui disappeared. Hinata granted her final wish.

2. We don't really know. It is never said in the anime exactly what happens. We know for sure that when you disappear you can be reborn into an animal or another person, but not much more then that.

3.

Spoiler

Hinata didn't disappear because while he granted Yui's final wish he has yet to grant his own. For lack of better words "His wishes have not been fulfilled."

Hope this helps!

 

but then, how did Hinata's actual self interact with Yui's actual self? Or did it at all?

 

That was the fantasy that Hinata told Yui to help her find peace. It was how Hinata fantasied Yui and him would meet if they were to be reborn. A "what if" sequence, I know it confused me at first as well.