Hi,

I see that you are pretty sure you are not getting power to the injectors circuits but have spark.
You have swapped out system relays from under the glove box area but have you studied the connector socket for bad terminals inside it.
There is one that goes to the fuel pumps that can get overheated the most.
Also there is one wire that runs off to a ground location that sometimes can go amiss on either end.

http://cleanflametrap.com/manIgnition76.jpg
This Shows a trouble shooting tree for a 1985 that goes up to 1988.

https://www.brickboard.com/RWD/volvo/1636835/220/240/260/280/now_issue_fuel_injection.html
This is a thread that discusses with some pictures from Art and lots of ideas that may help you.

What may be happening is one side of the relay is not closing due to a failure of it getting power.
Some people figure out which terminals are the main contacts terminals and put in jumper wires.
Too much work for me. I pull the cover off and wrap the contacts down with a rubber band or if out on the road once, a shoe string!

The 25 ampere fuse that was stock is a troublesome feature if not replace with a waterproofed enclosure. As I remember or read someplace it works directly with the fuel injector circuit.

I’m sure I can dream up other check points but I’m sure you don’t want me to lead you around more posies! Or (sup) posies.
(:-)

Phil