I could not find info on this unit, so am assuming it is electric not gas as you checked for a grounded element. Usually a G in the Model Number is for gas.

Perhaps the high limit thermo?
Possible but I doubt it, the unit should be able to run on high heat without tripping the breaker as long as the breaker is the proper size. Only way it could do this is if when it opens it shorts to the frame.

I would pull the heater again and recheck it for any arc/weld marks on the inside of the case.
Could be the heating element is heating up, sagging and touching the case.

Did a search on the Frigidaire site and here could find no info using the given Model Number. Then tried FGFL79 and all that comes up are stoves.

Cannot fault your logic in replacing the heating coil, I would have done the same thing. This one is going to be a pain.

I would try to get my hands on a clamp on current meter so you can monitor the amperage the unit is pulling. This will tell you if something is shorting or if the unit is pulling current that is too close to the breakers trip point.

Only other thing I can think of is perhaps one of the thermostats is opening and shorting to ground.
Try different temp settings and see if it trips ealier on higher temps.
You will have to let the unit cool down between tests for this to be valid.

I assume you have tried different timer cycles and times to rule out a timer issue.

trips the breaker at 4 mins on every heated cycle, runs all day on non-heated fluff air - so must be a heating issue, the only thing left are the thermostats and shorting to ground when open - right? As a final check I'm going to replace the breaker again on the off chance that I got a bad one off the shelf.