Sorry for your loss.
I think whatever broke it started with the OEM clutch or DMFW letting go. I known I've seen at least one other track-day clutch failure (ultimateracing 335. Another forum, tore the center right out of the disc) and two or more DMFW failures so far among the forums I visit.
1) It happened when you let off the gas. Sudden decel twists the hell out of the clutch and engine with no damping.
2) The clutch pedal went limp. There's no way I can think of a cyl dying would cause this. Only a clutch/slave problem.
3) If a cylinder went you would most likely not hear a sickening explosion. The vast majority of the time it's completely transparent except for a rough running.
4) If you really flooded the engine and smashed a piston at 5k rpm, that would most likely put a rod through the block or oil pan and spew fluid everywhere.
We won't know of course until it's torn down. My 3c.
2014 Audi S5 DSG
2011 335xi Sedan (retired)
2008 335xi Sedan (retired)