My mind is about to explode

With everything plugged in do you have a code 26 and 27?Or are you saying with prcm/passive restraint control module unplugged your getting a code 26 which would be normal since its part of the quad circuit. You most likely have a bad prcm which you call a auto seat belt relay.I have seen a lot of them fail and cause the speedometer to become inop. The speed signal goes thru the prcm and then to the speedometer the circuit fails in it and the speed signal never makes it to the speedometer. Lets try this put the front wheels on jack stands so the wheels have plenty of clearance from touching the ground. Also make sure the ebrake is on and the wheels a blocked. Then with the drivers door open start the car put it in gear then spin the wheels above 13 mph the power seat belt should close. If it doesn't then most likely you have a bad prcm let me know what you find.

The only problem with unplugging it is the speed signal goes thru the prcm so when you unplug it you really break the speed circuit. Try what I asked you to try and let me know what you find.