1) My car has 2 small amps in the trunk
2) driver side is a dual cone with whizzer(6ohm), passenger is oneway midrange(4ohm). Both have a 67mf cap which crosses them over at approx 600HZ on 6db slope. When I replace them with aftermarket, I will use a nice 100mf Mylar cap for around 400hz.

Door speakers sound like they are crossed over at about 500HZ and down with a coil probably. Nothing special there. I think the amp just runs juice to the speakers and has no internal crossovers. Simple to then add aftermarket speakers off the factory amp for time being

3) Yes
4) I am definitely doing this with fronts first, should be fine