Car power/start troubles

Rumble, some of what you posted is contradictory. It doesn't sound to me like the switch would do "all that" I'd do some rechecking tomorrow.

For example, you might have a bad connection through one of the bulkhead terminals, but "just enough" to operate your meter or test light, yet when you turn on the key or twist to start, the connection stops.

One thing you might do is rig up a "heavy load" test light, using a headlight. THAT sure as heck would put a heavy load on a bad connection and show it up. Or at least rig a test lamp using a stop/ tail lamp.

I'd go through and recheck the main feeds through the bulkhead

the ammeter

the "big red" feeding to the ignition switch connector,

and so on.

You said earlier you had no headlights. This alone indicates a break in a fairly "mainline" feed.

What's the history of this car, was it running at some not to much earlier date?