I don't know the set up for mowers but the wiring diagram certainly showed several options for a fault. If all the relays and safety switches are working my guess is you have an earth leakage in your starter motor. Like you said; cranking amps.It doesn't take much for a broken or faulty wire in the armature or stator.windings to leak to earth without throwing the circuit breaker. If you have that spare mower I would try changing the starter motor.