Most trailer wheel bearings are not pre-loaded or "Torqued". Common practice is to repack the wheel bearings, reinstall the hub and wheel/tire and first bring the nut up snug. Then spin the wheel a number of times to seat everything, back off the nut, snug down one more time and then back off about one flat until the cotter pin fits. That's worked on most tapered wheel bearings for the past 80+ years.
About wiring: Having a schematic will usually just make things worse as; a) Gardner Manufacturing (who built your Amerigo) was not well known for following any of the schematics they provided when building and, b) your trailer has been through umpteen owners in the past 40+ years who changed God only knows what in the wiring. Add to that any wire damage and shorts and opens and it's a lot easier to just pull out the old stuff and start from scratch, it's not that complicated.