But plenty of Jedi are totally irrelevant when the master of the Sith has manipulated the force so much that he alone is able to use it to perfection (or pretty much) and is superior to the Jedi.
Strong, yes. Superior? Very questionable. He was persuasive enough to seduce Anakin to the dark side, but that's more charm than power. He claimed he learned how to preserve life before killing his master. Sith lie, and we never saw it happen. We can't even argue Plagueis really knew how to do it since there's no canon books to back the claim up. All we really know is that he could make force lightning and he was cunning. However, Yoda and Obi were strong enough in the force to become ghosts.

Rey was there. She hadn't been there yet. It supports the Rey = Luke's daughter thing. He wasn't on before, but when Rey was in the area bleep bloop he's back. R2 was there to lead Rey to him not anyone else. Or that's my theory.
Luke told R2 to activate because Rey was there Both guesses fit together into a single solid reason.

OOC:I hate paragon playthroughs in Mass Effect. They're so obscenely candy sweet that it makes me sick. I do prefer middle of the road choices, but I'll take renegade over paragon.