Help me understand my timing please

No, work them "up" together, back and forth

Use as much initial time as you can without kickback on the starter. You can also get an idea by using a vacuum gauge, time at idle for max vacuum

Then adjust the speed screws "within reason"

Then adjust mixture for best/ highest/ smooth idle, readjusting the speed screw if necessary. On a new setup you may have to do this a couple of times

HAVE YOU determined TDC is correct yet?

YOU SHOULD consider what was posted about rotor phasing. There could be something wrong in the distributor mechanically or electrically. For example, those of us using GM HEI modules, if you hook the distributor pickup reversed, it will wrongly affect rotor phase.

You should at least consider checking rotor phase especially since you are having timing trouble.

And do some Googling about MSD and timing light troubles. I don't know if it applies to your setup, probably only to the digi MSD boxes.

If all else fails AFTER you have verified TDC scare up a known working ignition to put in there, even a points dist. or a Mopar breakerless system.