Was the INOP Airbag system disclosed to you on the sale of the vehicle to you? Likely not only has the airbag gone off, the indicator light has been removed or cut wires. This is a deep rabbit hole to go down to fix. I would take a close look at the driver side airbag for molded lettering 'Does not contain airbag' or other repairs looking like the bag was cut off and cover taped up.
Download Bobby Martins ds4-db2 conversion pdf. I will not own another ds4 truck. I didn't go back and read your electrical problems to see how much is related, but imo ds4 SUCKS. Let me count the problems with db2's, ok done, NONE.
One thing to remember about Bobby's guide is its for OBDI. OBDII is going to be a lot different as far as TCM hook up go's. Ive done it on OBDII and it took many months of research to perfect it.
Knowing what I know now would suggest getting an OptiShift trans controller http://leroydiesel.com/products/optishift-transmission-controller/
This will save many hours vs trying to put in a TCM.