So, in order to lose the tailed beast, would you have to have the contract broken, lose hypnotic control of the beast, have it sealed away, or all of the above?
All of the above, I'd say. it should in no way be any less riskier than having the beast stored in oneself (which raises a death flag for the duration of having it). Sure enough, one wouldn't be killed if their control was lost, though they could very well perish within the following few minutes.
Screw that, it would be easier just to kill the controller. Unlike Edo Tensei, the death of the controller would free the beast and render them unable to return summon until their eventual revival.
Now, as for dealing with the tailed beast after that, take it under your control and you're pretty much done. Still, I wonder what would happen if the master did not summon their tailed beast, and yet, they're still defeated in battle.
The contract immedietaly switch over?
*Updated, did I miss anything on the front page?
It would be the same situation as killing a host. No Bijuu for the person to take...
It was a suggestion about a scenario regarding what happens, BUT, the tailed beast would eventually be revived, and be up for grabs again. What route is taken for that I've already made a proposal for.
And I agree with Angra all the way, allowing enhanced tailed beast bomb/ susano'o biju is opening a can of worms. I'm not saying dumb it down to where everyone stands a chance, but if the only counter is to have the most powerful wood release techniques (or use space-time ninjutsu to dodge, but even then, you can't teleport an entire village without a squad), that rival Perfect Susano'o (if not greater in strength), then we are begging for village-wide destruction in a single post.
Combine this with perfect senninka, and you have two tailed beasts and a susano'o betwixt the lot of you. Overpowered can be relative, either through extreme convenience or overwhelming power to the point that it is a royal flush. But, many of us are on different levels and opinions, so while OP to me is even something like hiraishin, others would find it just fine.
And to be technical, the power to restrain and catch a biju
could be done with no resets, if you don't count secret techniques as resets, which most don't.