On some of these ( I can't see the parts ) the prop thrust washer can be used to turn the bearing carrier.----Left hand thread.-----These lower units are not robust.----I recommend you take it apart for inspection / reseal with a complete kit.
I can not see your parts !----Some did use 2 keepers.------Does your motor have the original lower unit ?----One piece lower shift rod ?-----Any pictures?
Take starter apart !-----Inspect test it.----Take ohm reading on armature.----If bad you have the smoking gun.----Guess work on outboards often does NOT work and gets expensive.
Put 12 volts directly to the motor.-----That would bypass lots of wires / switches / relays.----If motor works that way then work backwards to find fault.