The problem isn't with simply having fuel filling the plug cavity. I'm guessing your power panel regulates current flow so that when a plug is flooded it boosts the current to heat the plug enough to boil off the fuel but doesn't reduce the power quickly enough once it's clear. This overloads the coil and it burns out. I've been using inverted engines for more years than I care to think about but my power supply (with a manual adjustment) only provides whatever current is needed for a dull orange glow. If a plug is flooded I just let it cook for a few seconds to boil off any excess fuel or oil then it's ready to go.

The correct procedure for an inverted engine (or upright if it comes to that) is knowing exactly how much prime it needs and getting that prime spread evenly right through the engine. The hard part when inverted is that the fuel has to travel uphill through the carb to even get to the crankshaft port so usually it's easiest to flip the model over, prime a couple of turns then flick the prop a few times to get it spread around inside. As soon as it sounds slightly "wet" and feels loose then it should be ready to go. Put the model back on its wheels and give a couple of quick flips to make sure you haven't flooded it then hook up the plug power, wait for one or two seconds for the plug to get up to temp then one flick should have it running. Once it's running the idle mix needs to be set much more precisely than you can get away with on an upright engine but the 2-3 minutes that 2MuchThrow mentioned should be more than good enough.