How do I fix bobbin problems with a Singer CG-500/550?

This sewing machine was inherited from a former seamstress and has worked wonderfully for more than several years. These days, however, the sewing machine has been unusable due to problems with the bobbin and bobbin case.

It started with the needle thread wrapping around the bobbin spool while trying to straight stitch, which eventually caused the machine to jam up and not be able to sew at all. The instruction manual doesn't help at all with troubleshooting, I've reviewed a lot of Qs and As related to this problem, and I have tried solutions ranging from changing to a quality thread to cleaning out the machine (which all proved to be futile).

I removed the bobbin case to check for dirt and loose threads that might have gotten stuck underneath and now am having trouble repositioning it. I have followed the instructions for replacing the bobbin case in the manual, but they aren't very detailed and only guide me to a certain extent. I have tried positioning the bobbin case in different ways, turning the bobbin case to get it to lock in place (manual says it will lock), and checking to make sure it works by turning the handwheel towards me. It eventually jams again and this time there's something in the way of the needle when it goes through the plate because I've seen the needle bend and make a loud, almost-breaking sound. I'm afraid I've made the initial problem worse and I'm all out of ideas--please help!

Thanks,