Not good. It would be great for other games more specifically focused on fitness, but being able to walk the distance on a treadmill doesn't really fit with the game IMO, let alone the actual abuse potential.
I don't mind it the way it is except that it needs to be more stable/more generous if that's the only way to fix it, and also more transparent about what the speed limit really is.
Correct, that is not the case at all.
There may be an advantage to powering up an unevolved Pokemon if, say, you don't have enough candy to evolve them but could really use that Pokemon in attack/defense at that moment. I've done this multiple times. But there's no stat/power benefit to powering up before versus after evolution; everything has to do with your Trainer Level.