I think the "downgrading" of SYD-AY and ML-MIA from ejet to ATR has been on the books for a long time.
No idea on how the numbers stack up between Albury and Sydney, but there is no way that VA is making money on the ML-MIA run. Plenty of times there's been less than 50 passengers each way, and sometimes the 737 does the trip! IMHO, the only reason that VA are still servicing the route is to keep it open until an ATR can do it.
Mildura would require a MEL basing unless it came in from Canberra which is unlikely
Why so? CB-ML-MIA return isn't a difficult route. Qlink often did the reverse when the Q300 was based there.