I am sorry but STARTING a taming challenge should make that mob yours plain and simple it has nothing to do with being selfish. If you die you lost your chance better luck next time but until then its yours. Now its a different story if you haven't engaged the mob yet or waiting with others, First come first serve I guess.
The fact is.... it doesn't! So there's your answer from Blizzard, albeit indirectly. If they had wanted to do that, the mob would be untouchable during the time it was being tamed, this is not the case. The bottom line is, it's not yours until the golden ding, simples. Also, please do not assume I mean the same with loot roles as I do with mobs in the real world. They are two totally different things, so why make that assumption?
I feel the only person actually able to answer this point would be Noah himself. However since these challenges were put in the way they are currently, anyone can do anything at any time to interrupt, kill or mess with the creature. We hunters need to get down of our spoilt pedestal and realise rares don't belong to us or revolve solely for our enjoyment. You play with people, that means you have to interact with them and that won't always be on your terms. If you can't accept that then a MMO is the wrong genre of game to be playing.