I'm unclear about your goal... do you want a "simple spreadsheet model" for KSP, or for reality? In either case, I have serious problems with your FAA doc's "Similar to a rock skipping off a pond, a vehicle that doesn’t slow down enough may literally bounce off the atmosphere and back into the cold reaches of space" line, straight out of NASA/Hollywood PR hype. You don't bounce off; you simply fail to shed enough kinetic energy. Given metals' thermal conductivity, I think you could make a reasonable guess by considering work done (average drag [lots of work to be done here] x distance in atmosphere [most significant input, periapsis]) = Q into the metal = some coefficient times delta T. If T > melting point, game over. That your metal lumps don't survive is probably why NASA doesn't use bare metal for reentry shields
I don't like the tone, but I haven't found serious problems with the math. I think a high level energy approach, as I described above, gives you the easiest "back of the envelope" estimate. May do a political rant elsewhere on why the FAA is spending my dollars writing this stuff.