Extremely random, weak spark; sometimes no start

No, the Pertronix Flamethrower coil by itself is just a plain coil.

OP, the changes in voltage at coil + and coil - with the key on but not running indicate that the voltage supply to the coil and points is erratic, or that there is an erratic partial short around the distributor.

The supply side issues can be in the key switch, the connectors to it, the bulkhead connectors, the ballast being erratic (not so likely) or any where in the wiring from the key switch to the coil +.

The erratic short on the coil - side could be in the wire from coil to distributor (shorted or open) or the condensor.

Also, be sure that the points are not closed or closing when you do this test; the 6.6 and 1.1 volt readings at coil + and - are close to normal when the points are closed. So that test info could be OK, depending on how you made the test and if the points are closed or open. (I've seen this voltage look erratic if the points were just on the verge of closing and just breathing on them made them close or open.)

And your last running voltage readings at coil + and - look to be pretty normal. (I will guess you are using an analog meter....)