Hey Jim,
Interesting thread...I've enjoyed following your progress! Here's my 2 bits...
I really doubt you have a flywheel issue. It's possible, but I would put it lower on the list of things to check first. With all the starting you have been doing, I would check the battery charge and the solenoid first. The starter uses an inertial engagement, so it has to start spinning hard and fast to drive the gear out on the nose to engage the ring gear on the flywheel. If the battery is weak, or the solenoid is old...it can reduce the amperage available to spin the starter fast enough.
This is easy to check. First, charge the battery completely and try to start the car again. If that is a no-go, then just disconnect the batter cabel from the solenoid and manually drive it to the stud on the solenoid going to the starter. You are manually bypassing the solenoid (be careful and use gloves). If the solenoid contacts are oxidized, this will get the engine turning. If it does, you have isolated the solenoid as the problem.
The next thing I'd try is to manually rotate the engine 1/8th turn or so and try to: A) listen for the flywheel ring gear rattling as the engine turns (very NOT likely) and B) Try starting again normally. By turning the engine, if you are missing a few teeth, this will align the starter with good teeth and should allow at least one good start.
The next thing to check, in the order of difficulty, is to remove the starter and check it on the bench to make sure the gear drives outward when you first apply 12 volts to it. If it doesn't, it may just have a dirty snout and need some cleaning. Clean it up and try again.
If these fairly easy items don't fix it...then it's time to get under the car and look at the flywheel teeth. I truely think you would have had some indication if your ring teeth are bad...like skipping and banging during the starts. If the ring gear came off the flywheel, you likely would have heard some awful noises!
Now for my 4 bits on the backfiring. As I see it, there are 3 possible causes:
1) Sticking valve...rule that out, as your idle is too consistent and it would backfire frequently instead of only on acceleration
2) Plug wires reversed...easily checked. Note...WAY too much advance can cause the intake backfire. A bad plug or wire will not.
3) You are lean. For the lean, first use at least 30w oil in the dashpots and pull the choke about 3-4 clicks...then repeat your experiment with the acceleration to make it backfire. If it stops backfiring...you need to find a way to richen it. Could be a mixture issue, but also could be a vacuum leak.
Personally, I would not be concerned about the rich looking plugs. If you have a lean misfire, that can also soot the plugs and fool you. Once you get it to run without missing or backfiring, then the plugs will tell the true tail of what the mixture is.