Wow. Great pictures, Matt. The backfire out the carb is because that valve doesn't seat. Valve doesn't seat because there is a piece of something big in it. The "something" banged around a while and damaged the plug before it got stuck. The gap on that rocker is because the valve won't seat.
The red on the plug is OK. That's just addatives. probably from running the HEET for 20 minutes.
Now how you got mechanical damage in 3 cylinders at once: I don't get it. I guess it is possible that when the valve got stuck open chunks from the bad cylinder got pushed back up into the manifold and sucked down another jug.
No matter what, you're pullin' the head on this one. With that much damage, I think you'll find some beat up piston heads as well as the head damage. Sorry. It may very well be time for a different engine.