Before pulling starter, I would check the engine ground strap.
You can have 12v positive at the starter solinoid terminal 50 with the key in the Start position , verifying starer voltage , but both the starter motor and the starter solinoid need a ground to complete the circuit..if no ground, you would have the exact complaint you are having.
An easy test for this is to put a jumper cable from the Battery neg to the engine block and retry the start procedure.
I only mention this b/c you say you jumpered the starter direct across the load side terminals of the solinoid, [ if I read you correctly]and that would spin the starter even without the solinoid. But it would not with a bad ground ...