I hate to throw a spanner in the works here, but unless (1) you have genuine remote key fobs which means that an up-graded system was fitted by Volvo at some point (probably when new), or (2) an after-market immobiliser system has been fitted, then you are looking for something that does not exist.
As far as I know, the 480 never had an immobiliser fitted as standard. Certainly I have had a number now from 1988 model to 1995 and various points between, and none had an immobiliser. Following the 1993/4 facelift the 440/460 gained a remote fob system on some of the higher spec models and this included an immobiliser, but (I guess due to the small numbers being made by then) this was never added to the 480.
The standard set-up is key-operated central locking and is pretty basic. This arms the alarm and the red LED flashes. The alarm (single horn note) will be triggered by opening a door, boot, bonnet, or fiddling with the ignition:
Unlock the car with the key from the driver's door and both doors unlock and the alarm is deactivated, and the red LED stops flashing.
Unlock the passenger door with the key and the alarm is disabled to that door and the LED stops flashing, but ONLY the passenger door is unlocked. Reach across and open the driver's door from the inside and the alarm will go off.
Unlock just the boot with the key and the alarm is deactivated to the boot only. The red LED remains flashing and opening any of the doors or bonnet from inside will trigger the alarm.
I can see that you have - quite sensibly - been looking at 89/480's thread about central locking, but the behaviour he described with his car is very strange to me; it is well out of line with the above and sounds almost as if someone had swapped the systems over between passenger and driver's doors, or something. I do not think that post will help you.
So my suggestion is that if your car won't start, the problem lies elsewhere - there shouldn't be an immobiliser to be causing the problem. Have you read the fault codes from the box reader on the left strut pillar? Otherwise if you can give a bit more detail about the problem I am sure between us we can help!
David