i dont think the "rotation" of the pouch had to be gradual, it could have been a single mutation like when people get a mutation that makes their legs face backwards.
The question is advantage of a backward facing pouch or disadvantage of a forward facing pouch. And what came first? And from what it evolved?
My guess and its a guess i dint read up on it is that the pouch was originally backwards orientated evolving from sexual organs or something similar in the back part of the body.
A ken-guru with a pouch facing backwards would probably have a big problem holding on to his young while jumping around. So my guess is the freedom of jumping around was first enabled by the rotation of the pouch giving the animal more speed while carrying their young giving it a better chance for survival.
All the back pouch facing marsupials never had any selective pressure to turn their pouch around so their pouches stayed facing backwards or at some point even had selective pressures to keep their pouch facing backwards like digging and getting their pouch messy in the process.
This argument is pulled out of my arse so it might be wrong.