Does this have spoilers for Tsukihime -A piece of blue glass moon-? ☴

Yes.

Where does the community primarily play at? ☴

PC is where a majority of legacy Melty players are, but PS4 should be fairly populated as well.

Can my computer run this? ☴

The minimum requirements are listed on the Steam page but you can also check whether or not you can run it at Can You Run It

Does this game have rollback? ☴

Yes! The developers have been very vocal about what they need help with, too. It will only get better with time.