After considerable thought and some feedback from Charles, my working theory is that suffocation was the cause of death of the fish. I couldn't find it in any of the articles or instructions that have been linked to, but apparently flub is an "oxygen scavenger."
I think what happened is that the filter on the QT (a Tetra internal model, I think it's the 30i) provides adequate circulation to aerate the water for the fish, but not enough for the fish+flub (especially at higher dosage levels, which leads me to the next point).
I also found out from Charles that the flub instructions linked to on wkndracer's QT thread are outdated, because since publishing them he's switched to a different supplier and is now selling flub in a 10% concentration, rather than 5% as before (he's currently working on an updated set of instructions to give out). Because of that, I ended up dosing my tanks at approximately 2x the normal amount (he mentioned that it's difficult to OD flub because it drops out of solution quickly when the temperature drops).
My main tank is still fine, with clear water and healthy occupants, but the oxygenation rate is substantially higher since it's only half full and my canister outflow is spraying against the back wall of the tank about 6 inches above the water level. The QT however, since it probably only had adequate aeration to begin with, wasn't able to cope with the oxygen scavenging level of a 2x flub dose (BTW, both tanks are kept at 78F).
So, the moral of the story: when dosing flub, ALWAYS use an airstone (even if aeration is normally adequate). Charles, please include this point in your new instruction set.
I still can't explain the white stuff, but wonder if it might be related to a mass death of the nitrifying bacteria due to the O2 levels.
I'll keep you posted on the progress of the main tank.