..."I checked "under the hood" and found the tire had worn off to a flat surface (how did *that* happen?), got a new tire from another machine and ..... that didn't help either."
Well, there's a question that i think I can answer! The tire is probably rubber, and if the tire is set in one position for a long time, it flattens where it 'sits' against something. My guess is that the flat spot is what is causing the bobbin to wind improperly. My guess is that if you can replace this little tire, the bobbin will wind correctly.
How do I know this stuff? My husband repairs and collects 45 record players! Many of them have rubber wheels in them that are flat from sitting in one place for years. In some of them the rubber wheel is cracked and split as well. Either way, they need to be replaced.
Where to get a replacement for your sewing machine - I have no clue.