Sinse I have been following this I have been greatly entertained!
Point of recomendation:
I would seriuosly consider runnng a 10 to 15 uf PFC capacitor on the output side. This will round off that square wave and give you a more accurate voltage reading.
It wont be exact but it will be closer.
Plus when you switch off those mosfets for that 25% time you will get a large voltage spike that needs to be handled some how. And I would agree that it does have a sort of flyback effect due to that magnetic field collapsing and there being a fair time delay before the other side is switched on. That magnetic field energy has to go some place durring that off time or it just builds up voltage where you least want it! Namely at the mosfets.
The PFC capacitor will take some of that spike and help recycle it back into the system creating a LC tank effect and greatly reducing the idle current as well. But still, the primary is going to get a spike that could be fairly high when there is no actual load on the output.
A lower voltage AC capacitor of perhaps 1 - 3 uf per running amp in series with a 10 ohm or so resistor connected across the ends of the primary will greatly help protect the Mosfets.
I am just guessing at the capacitor and resistor values do to the fact I do not know what VA rating and core saturation values that transformer may have.
But I do know you are still far better off with them than without. reguardless of if they are sized exactly correct or not.
I have built enough homemade "DR. frankenstien" inverters to know these added components will greatly help in the overall duribility and running efficiency.
It is still a crude unregulated design but it does have potential. And I would shoot for a 15% over line voltage number at no load. You are not going to get away from the voltage sag with load increase on this type of circuit.
I would not be the least bit surprized to see it drop to 15% under voltage or more at is maximum rated capacity too!