And what proof will we recorded in stating whom had what bijū and if that individual achieved 'mastery' during their stint as a jinchūriki?
I don't mean to get off topic but the answer to that question will determine my vote.
That could be included in the challenge threads I suppose, though other than that burden of proof would be on the person wanting to claim early mastery. Proof that would count would be a match here on the forum where the host had mastered the beast or an old challenge thread. Naturally any masteries before any biju fights were on the forum at all would be next to impossible to verify, and so those foggies would have to go through full mastery again.
Then that individual can simply lie about their previous mastery with it is what I'm getting out of this.
Rather, I think there should be a locked thread that details the jinchūriki as they pop up and with whom had mastery that only the bijū council can't modify as hands change over with each beast.
The person wanting to claim early mastery would have to provide proof that shows that they had previous mastery over the beast. Lying in the proof I guess is possible, but then again, screenshots of PM's have that same kind of risk. What proof would have to be given in order to get on this hypothetical list? The same kind of proof that would be used to verify if the person had previous mastery, most likely past fights, challenge threads, or whatever the Council accepts as evidence in that scenario.
What qualifies as proof though can be another topic. This one is strictly on what the reduced time is, as already it has been established that a reduction in time is permitted.