Saw it again. Liked it a lot more, and I’ve made peace with Luke’s death.
I don’t think he “willed himself into the Force,” but I do think he knew what he was going to do would kill him, and he sacrificed himself willingly.
And yeah, Force spirit Luke is definitely haunting Kylo in IX. It would be a huge missed opportunity if that doesn’t happen, especially since, as has been pointed out, he flat-out tells Kylo that’s what he’s going to do.