I read some articles about how mosfets and diodes work, and I've ruled
out the high-current part of the circuit. The mosfets get the source
current from the battery, and the drain conducts just fine to the
negative pin on the motor contacts (the positive pins, in the middle,
are linked together).
However, the gate voltage is equal to the source voltage, and doesn't
change as I move the sticks on the transmitter. This makes me think the
fault is somewhere in the receiver circuit, as it appears to be
ignoring any command that would cause the motors to run. I tested the
transistors that drive the mosfets and they test fine too, so it's got
to be somewhere beyond that.
I've no idea how a motor overload could have caused this. It doesn't
seem likely; too much current would have blown something in the high
current part of the circuit, not something in the receiver.
Maybe it was the crash, but none of the components seem broken and/or
desoldered due to physical shock... and it didn't quite crash hard
enough to cause this anyway, after all it was on the bed when the
accident happened.
I'm all out of ideas.