Yes, there should be a reset bounties button, and a close Grift/rift button (however, the later one could have big repercussions on Public Games)
It was mentioned on the latest PTR feedbacks, but nothing really happened. So far.
You do realize that there is an option to join or not a boss fight, of course that in multiplayer games all players have to agree to close the rift for it to close, otherwise it will be chaos. Blizzard is lazy but not stupid!
There is no limitation. It would be in bronze age ;D I rather see reason as "they can just exit the game, right? so where is problem".
Are you sure about that? The problem with simply exiting is that you lose time and after a while it gets annoying. If they can implement it and don't want it, then they assume this will deter grift phishers from rolling endlessly until they find a perfect one, which makes no sense in the end, because they still do so by exiting and creating a new one. There is no real response on this matter from Blizzard, all I could find is a reply from Grimiku back in 2014 saying this is a good idea to test on the then ongoing PTR and would mention it at their next meeting. Nothing so far. The reason they put the bonus cache in the game is to give players more reagents and cut down on the runs required to gather them, therefore easing on the workload for the servers. This is another good option, when players are creating useless games just to test a grift. That's one of the reasons in Diablo 2 they limited how many games you can create for a period of time.