Thanks for the detailed reply!
So far i havent felt it default to FWD, its been pretty consistant in AWD from what i can tell.
My concern is that the difference in tires will damage the drive train or something, but atthe same time, i dont want to fork out for 4 new tyres as these are in excellent condition!
Not too sure of the exact tread depth, but both tyres tread are identical (at most 1mm out from one another)
Cheers!