I understand your first post as "on USB connection, my player will reboot and start in the original firmware, only the original firmware doesn't register the USB connection". If the original firmware starts correctly in this automatic procedure, then you definitely should be able to do it manually too. There is no need to restore something like in the thread you linked.
Have you tried my suggestion and booted your c250 into the Sansa firmware manually (while it is not connected) and then plugged the USB cable after the original firmware is up and running?