My bad on the Mini bits, sorry about that!

Stepper motors need phased power to all 4 wires. I doubt very much if the motor has gone bad, as they are very reliable.

I've found this thread :https://www.rcgroups.com/forums/show...inter-thoughts regarding the printer. Having had a scan through, it's as I thought, your printer uses a pretty standard board, but you would need the firmware. I guess the people who posted on this thread might well be able to help with that.

They mention issues connecting the USB and internal power at the same time and also that the regulator on the board goes bad - which would explain at least some of your problems. Sounds like it's a good solid machine though, so worth tinkering with, even if you end up replacing the main board.