The HEI should start with 9 or 10 volts, they like full voltage but still should start when low. Your voltage at the purple wire shouldn't have any effect on how it starts. All this wire and voltage is doing is pulling in the starter solenoid. It has nothing to do with the HEI or ignition switch. You proved that with the remote solenoid.
Before changing a lot of stuff again, try this when it won't start. Hook a wire from battery (+) to the HEI coil, then use the key switch to crank it. Do nothing else!
It better start if it cranks, if not, pull the modual, trash it, and get a GM unit!
Again, go from battery post to coil only!
Just for kicks, once running, pull and tug on the wires fron the coil to the modual. Also get the vacumm advance to pull in and out to check its movement. Each time it moves, so do the small wires inside. You might have a weak wire at or in the HEI itself.
Another thought, when it acts up again, don't do anything to it other than hooking up a battery charger with Booster/start mode if you have one. By adding more power all around it should start.
I don't know what this will tell us other than it needs more voltage, which may lead back to the modual being weak or bad.
Joe
