When you bypassed the power relay did the engine run?

This will suck if it does end up being a bad ecm thats not available. Ive never had much luck with sending controllers and ecms out for repair.

From everything you said it sounds like powers not going to be your issue for the most part and need to focus on the whole ground side. Engine grounds, ecm grounds and go from there i would even bypass that stupid battery disconnect and run my own fresh ground wires for the ecm to rule those out as well.

I hope your able to find somthing, all this work you definitly need a win!