This is, for me, at any rate, the peculiar joy of a series. No matter how bad it is, no matter what the previous ten owners did, it can be fixed by me. And, if I do it right, the repair will last for longer than the life of most modern cars.
Right!
Good progress. Get those diagrams, well make life easier. Remove all the fuses again.
Put one and only oneback. And see what happens. See what works. Now you know what that fuse does so take a note.
Keep going until you get the spark again. That's a broken circuit.
That's good work @uytter . Well done.
This is all from your photos so I'm not 100% on any of this advice but.....
The valve looks like an on-off switch. An electrical isolation valve, if you wish.
Keep the positive terminal of the battery set isolated, don't attach the positive cable for...