Loss of power on one bus in main panel
You should check for 240 between lines, but I'm guessing you have it.

When I turn on any circuit breaker connected to the one bus line the voltage to that bus and to the incoming supply line drops down to anywhere from 20 to 2 volts.

That smells exactly like a very bad connection upstream on that leg. No current, full voltage. Turn anything on, current flows, high resistance in the bad connection, voltage you see at the panel input drops.
Given it's on the input side beyond your panel input, I think it's call the power company time in most jurisdictions.