It seems to me that when you bottle or keg carbonate with sugar the yeast would scavenge the small amount of O2 introduced in gelatin fining.
I've wondered about gelatin's ability to settle out at a later time, but have never tested it. I know they say that if you use gelatin, and crash it to promote the sedimentation, that if you disturb it later on then you the gelatin may not sediment again very well (at least I've read this a few times before). I also read that isinglass is both a better fining agent due to a better collagen structure (or some such thing), AND that it will more readily sediment after being disturbed (again, just something I've read).
So what about the scenario where you keg with priming sugar AND gelatin. You would need to keep the keg at room temp for 2 weeks for priming. At this point, I assume the gelatin might still not have done it's job of fining and sedimenting, and that once you get the keg into a cold environment then the gelatin fining will finally take effect, but I don't know?