Yes, the B450 Aorus Elite is suitable for the R9 3900X.
Yes, running R7 3700X at stock clocks with PBO Off should work fine.
Aim for an improved setup.
Sure, lower the voltage a bit and add some fans for better performance. A better GPU might still work well with an R7 3700X at that setting.
Aorus Pro fell short. The Elite offers a more affordable, budget-friendly alternative.