Whew! I put in some time today and cleaned and re-oiled everything. I added heat which helped a bit underneath in the bobbin area. Here's a tip: Don't pick up your heat gun from the wrong end..ask me how I know.
My 500 still wasn't running well and the motor was really noisy. I had tried the motor oil trick and that didn't make a difference so I pulled the motor out. I checked the brushes-they were fine. I checked the connections-they were fine but I cleaned them anyway. The motor looked good. Since I had it opened up I cleaned the commutator.
When I was putting it back together I noticed the end piece didn't turn well. It felt choppy and sounded like I was crunching potato chips. I tried taking it apart and reassembling it a few times to be sure nothing was in the way but that wasn't it.
I figured the bearing must be shot so now what? i don't know if this was a bad or good idea but I sprayed the end with Lectra-motive. It dissolves greasy gunk really well and immediately I could free spin the end! I carefully suspended it to dry in a way that the cleaner wouldn't drip into the motor and then put everything back together. That seems to have done the trick! It's much quieter and it's running smoothly. I guess now I have to give it the test of time to be sure I haven't done any damage but it's working really well now.
I fixed the broken tension with some non-original parts added to the pre-existing ones. The poor baby has no lid but I'm ready to move forward and see if it sews! Yay!