This will most likely need a wiring schematic or even a few tests to prove out. The high pressure pump is the issue, as it's PWM. The high pump has pulse either + or -, Traditionally it would be -. To solve this, you could power the whole shooting match with one master relay and then set it to kill by the inertia switch. Sorry, I never would have guessed this to be so hard. Before getting super creative, why don't you see if their tech support can help. that way you get all of everybodies answers before redoing it twice.
That's plan A...
I got it your working on Plan B
We may be well beyond that by now... but the install is getting safer simply by power of thought at this point.
I think your heart and head are in the right place on this. Try to make your mods as safe as possible and I can see the seat belts as a win too. At some point you just have to accept that you are dealing with the safety of an older car designed for a different era. You're far better off than the 50's car, and before you go too far trying to upgrade everything else...
Get a better data sample with another couple tries before you panic. Fuel quality, sitting time, temperature and just plain old error could be factors in this. Focus on how she drives for now, later on read the spark plugs to decide if she's running rich. Your A/F sensor will read everything it sees as coming from the cylinders, so any exhaust leaks are bound to throw it off. Check for leaks by using gloves or rags and try to hold your tail pipe shut at idle. A sound system will push away your hand(s), a leak will usually start to whistle a little so a helper may be needed to find out where it leaks.
