[RIP]

It's impossible to give specific info without knowing the process your mechanics have gone through to lead them to a bad computer. As I see it you have two options. Get the van to competent mechanics or get the van back and work on it yourself.

Have you tried your local dealership? At this point that may be your best move. I normally avoid them but, in your case there's been so many eyes looking at it with no results, the "dealerships are best equipped to handle tough to fix issues" train of thought may apply.

If you have a handle on basic auto stuff or know someone who does and is willing to help then come on back here and we can guide you through the process. Far from a sure thing but, we'll try to get it running. First move if you try this is to buy a maintenance manual.

You said they changed a leaking fuel rail. Did they tell you there is a safety recall for leaking fuel rails on 96 Voyagers? It may or may not apply to your van specifically but, worth looking into. Might get rembersed.

What engine do you have? Did this all start with a starting problem or just a leaking fuel rail? Did they change the camshaft sensor? Were you getting a Service Engine Light (SEL) on the dash? Did the mechanics ever say anything about the Automatic Shutdown relay or system? Cheers.