We can fill 30 pages of what ifs.
If you want answers, a bidirectional scan tool would be hooked up. That’s not a little gadget that tells you codes. They are far more than that. All parts of the network are looked at as a network map.
Then begin the slow task of back probing circuit after circuit based on results from the scan tool and looking at wave form data on an oscilloscope to make sure everything is up to scratch.
Have we got an answer on the blue stuff on the battery. A photo?
Have we got verification of a load test and battery health test results?
You can buy a relay tester. It’s one thing to use a multimeter on a relay. Testing its operation under repeated loads with a relay tester is another thing. They cost like $30. You can get them on eBay. They don’t need to be fancy. They provide a straight out yes or no after a test.
Everything else is like weeing into the wind.
An auto sparkie worth their salt would no how to back probe and use an oscilloscope. Ideally you use a diagnostics tech. Someone who
specialises in vehicle network diagnostics.