Re (1): I believe "Why first paint" already answers this... Yes, in isolation it's not sufficient to understand the full picture, but when combined with other metrics it helps developers understand how their pages are being painted. If the browsers change their heuristics, that's good to know, as I might want to update how I construct + ship my page.
Re (2): as in, record the time of first rAF and call that first paint? Correct me if I'm wrong, but this wouldn't account for the cases when all you've painted is default background color? The intent behind FP is to refine that and make sure that at leas something other than default background is on the screen.
Re (3): It'd be helpful to break down what edge-cases these proposals would actually solve. As in, if we round everything to next rAF, what are we accounting for that we didn't before, what did we regress, etc. I don't have a clear picture of what these cases are, so it's hard to make a call one way or another.