I am on and off here as a part time assistant, we have our own shop as well
The motor that drives the Up/Down may be this issue from what you say, however I still think there may be trapped thread around the Takeup Crank causing the stiff handwheel and adding strain generally... this will give a false reading for the Needlebar and the grating noise from overworking the motor gears
See attached for a view of the removed Crank Mechanism so you can recognise it...
Try a simple sew test and manually wind handwheel if directed... will it stitch anything, or is it trying briefly and stopping ?
Takeup Crank is on the Top shaft... if you open the L door (aka Face Plate) to see the needle arm, it is behind that, the offset arrangement of armatures that move the small arm up and down to draw thread for each stitch
Access from above is quite easy too... see attached
I need hardly say ALWAYS unplug first !?!?
Just wonder if you are able to attach images on your side... we can, but not sure if you can ?? Not that I need one, just interested for the future as I have only recently started this at JA again after a 5 year break