Seems fair.
Maybe the past host (assuming their character's dead) could control it if a GM is unable to do so if they've managed to master more than half of its tails when they were alive?
I'd think that if 50%> tails are mastered, then there'd be a somewhat friendly relationship between beast and host, enough for the beast to hold a grudge against those who partook in their death and favor towards those allied with the ex-host (meaning there'd be contextual bias depending on the deceased host's relationships).
Challenged and host are all the same. Anyways, the point of it is, a GM would only be required to control it if either the host abused their privilege or if the host and the challenger could not decide on someone to control the beast.
Controlling and mastering the tails is different than bonding with the beast. The latter is very subjective, but the former can be done without ever really bonding personally with the beast.
Yeah there is no way to measure how much your bijuu loves you though. So we should just leave it to tails.
Or, we should leave that out altogether, since, especially in recent times, not everyone is totally honest about when and how they master their tails. I'm not saying post every minute of it, but I know some people who have mastered their tails, all of them, day one.
I feel it would give too much leverage to the challenger. We are bound to have more than a single challenger with these RP rules in place, so if the host could always, no matter what, control the beast after their respective deaths, then in a tight spot, they would just allow themselves to be killed (rather than fight for just serious injury or complete dodge) in order to get the chance to control a wild tailed beast.
After all, if they were to kill all the challengers while as the tailed beast, they would remain the beast's controller/player for the remainder of that beast's free time. That's a power that I think shouldn't be tossed so easily into the host's hands.