Since you know that there is a place to jump at the starter... why not install a permanent starter button inside the car. This is the same type arrangement except with better contacts that one would use to bump a gas engine to turn it small amounts ( with dist disabled..... would not do that with diesel of course since it would likely start. )
In the old days cars had a starter button on the dash. Seems like until your multiple car gremlins quit messing with you this might be a good solution. This would give you breathing room for finding the actual problem while giving you comfort that you could use this car for parts chasing.