Sniper and Trunk Battery help

I still think the solution overall is to scrap the one wire. If you break the output at any substantial charge rate, with a Painless/ Moroso relay installed, AND I AM ASSUMING that these have no "load" protection in them, then the alternator itself will spike, and it may just ruin some components in the alternator......like the regulator

You MAY be able to easily "break into" the alternator and modify the field circuit so charge can be stopped in the field / regulator, but that is iffy

I am not at all in favor of a dedicated charge wire to the battery. (No relay). This LEAVES a substantial wire "hot" with the disconnect removed.

My "strategy" is this:
Use a 4 terminal disconnect. Use the small terminals to break a continuous relay FED BY the ignition switch and those terminals in series. Use the contacts of the continous relay to feed your ignition switch switched loads AND break the regulator circuit. This kills the ignition / fuel pumps/ etc as well as the VR circuit. The "3 wire" alternator then hooked to the "loads" side of the disconnect kills the charge wire from the battery

With the above you can also use a dedicated Ford starter relay if you wish.

You seem to be confusing the Ford type relay with continuous duty. The Ford relay is ONLY for a dedicated starter circuit, nothing else.