Looking at your schematic, it looks like the the neutral start relay must be operated and the starter lock out not operated in order to apply a ground to term 75A of the start solenoid. The starter lockout may be operated by a fuel pressure switch to prevent the starter from operating when the engine is running.It looks like grounding term 75A on the starter solenoid would bypass all the safeties in the ground path. There may well be other switches/relays in the battery path from term 16A, but I can't tell from the schematic.
You cannot tell for sure that you have 12 volts on wire 75A without disconnecting the wire from the solenoid. You could be seeing back feed from term 16A. Disconnect and check for voltage on the wire, should be zero or ground.