What is this brake line fitting called

Rear brake line/junction block...IIRC.....

Yes and yes to the other two questions.

Yep...^^^. It's really nothing but a Tee fitting to send the fluid to each side. As long as it's not rusted or overly corroded, you're good.

As far as the rubber line that attaches to it.....If it's still flexible and not "hard" it can be reused. But with that said, that hose probably will only cost in the "teens" to replace, so while you're in there, it wouldn't hurt to just go new and forget about it for as many years ahead that I will be alive.:D
Oh, and I'm 55...lol.