The 50/50/50 rule applies only at UQ, that I know of
Even if you get 100th percentile, you won't get in unless you get 50/50/50
But anyway, my prediction on the UMAT for this year would probably be higher cut-off for the Main Interviews, purely because there's less spaces, but I'm not a huge fan of the whole new ATAR/UMAT stream, it sounds a tad bit far-fetched to me, and I think we all feel the same way about anything after the Main Round being "dodgy" in some sense
I think that if they do intend to have a stream to cater for those with high ATAR's yet low UMAT's, they should just have an ATAR + Interview, and disregard UMAT, possibly reduce their 99.95 for the ATAR stream down to like 99.75 and chuck an interview in there
With UMAT, there's always those that are a fan of it, because doing well on it compensates for doing well on the ATAR, and there's always those that hate it because they did badly on it and now they won't get a place even if their ATAR is good.
So, the only real way that will please everyone is to have two completely different streams, one normal one, and one based on ATAR/Interview only.
My suggestion is to make all the Bonded Medical Places into ATAR Only - that seems reasonable - it's about 1/3 of their places