IIRC the first EFI systems didn't have a Schrader valve. In fact, here's a search for "early schrader valve" and there are a couple of hits.
And the logical place is near the regulator, but if yours doesn't have one there then I'm guessing yours is an early one.
Some of the early EFI rails didn't have a schrader valve, so that may be what you have.
To pull codes you don't need a reader. Go to Documentation/Electrical/Electronic Engine Controls (EEC) and then the tab for EEC-IV and the Checking Codes tab.
But, you could be right about a component breaking down. However, anything that would do that would likely be in the ignition system. Have you checked for spark when it runs poorly or doesn't run?
Schrader valves weren't installed on the early EFI systems. I think mid-year '86 is when they arrived. I think.
I know you said you have 6v coming out of the inertia switch, but you also said "where the others gave me anywhere from 10-12" and I don't understand what that means. So let's take...
... Schrader Valve then maybe there should be one? Ditto, or pressure test port? Is there a way to put one, or these, in one of the new lines?
My preference would be to order the lines already "made up" than to do it myself. If I didn't make it up well enough and there was a problem, it would be a lot of...