Since it use to start and the issue was turning over slowly, then it would turn over but not start, and now would not turn over. Whit that we have temporally ruled out something being wrong with the motor, and finding rust on the starter helped. Just guessing but figured the starter was on its last legs and also not getting enough juice due to bad cables. Now that it gets better connections and the starter gives breaks or the flywheel teeth do.
It could still be you have a motor problem and the starter could not over come the resistance until something broke. If the motor never turned over we would have said pull the drive and pull the plugs to find out if the drive is dragging the motor, or there is water in the cylinders. Water in the cylinders usually stops the motor from turning over and that's it, it's locked up. The drive may be low on lube or have water in it and this causes a drag.
So right now were just guessing from what were seeing. After the starter spinning fast starts me thinking about other issues, but need to get the starter off to see what happened. Might also be more corrosion as to why the key would not spin the starter
If your up to it, you could pull plugs and have a look. If plugs were pulled you should be able to turn the motor over by had. If not able to turn by hand, then pull the drive and try again.
All depends on how much you want to try before changing the starter. With it being so hard to get to the starter, you might even want to pull the motor to replace, but probably at least want to remove the exhaust manifold on starboard side to give some room
