It sounds like you’ve done a lot of homework already, and honestly, a lot of these motor controllers do expect some trial and error if the documentation isn’t very detailed. Your understanding of the load compensation adjustment seems correct — you basically tweak it while the motor is running...
No — that won’t work. Paralleling a 9V and a 3V source to try to make a 6V, 10 A supply is asking them to do something they weren’t designed for: their voltages don’t match, so they’ll fight each other and dump current into one another; a typical 9V or small 3V pack can’t supply anywhere near 10...