Bi-amp them to me is best, then you have control.
I used our KEY amp, then I can always add a bigger 4 channel amp, keep DSP and time alignment and use KEY as pre-amp.
So you run the 3.5's off the Key Amp, and the F/R door speakers off another amp?
In my case I have RD400 4 ch - Front Ch. running the Front Door/Dash combo (since this is how Ram wires them) and Rear Ch. running the rear doors. Then a dedicated sub amp.
I'm running the stock 8.4, and am getting ready to swap out my Metra DSP for a RF DSR1 later this week.
So in your example, I would split the Front Door/Front Dash combo, use the RD400 to run Fr/Rear doors, and then use the Key amp to run the dash speakers, if I am understanding correctly? Or any 2ch amp? That would resolve the ohm issue, and would give me control over the dash speakers separate from the front doors?