I put in a new needle, tried the bobbin case and bobbin from a machine that does work, and there was no difference.
I have watched the first three videos on the tension, and am thinking it could be involved because the dial doesn't turn from 0-9. I will watch the fourth video, then be brave and take apart and put back together the tension and see if that works. Probably not tonight, though... And it will be fun to try and also to clean the assembly better.
It doesn't look as scary as I had feared, I love the videos, Tammi! Fun to hear your voice, too. Thank you.