Alright, I am happy to finally report a qualified success. While it did not balloon up to any great volume, (about 1/3 of an inch from the rim of a 2 inch pan) the cake did not collapse at all during baking, and the layer is almost perfectly flat, with only a very gentle dip toward the centre. While I would have liked to see it reach the rim, this is definitely within the normal range described in my books for Genoise.
I only call it a qualified success because I still do not know for certain what caused the original problem. There are three things I did differently this time, and therefore three possibilities:
1. Mixed about 2 tbsp of batter into the melted butter prior to folding it in.
2. Preheated the oven with the baking stone for nearly an hour, as opposed to before, when I only preheated it until the thermometer reached the desired temperature. (about 15 minutes)
3. Used my Five Roses Cake/Pastry flour, as opposed to Oak Manor organic cake/pastry flour or Swan's Down cake flour.
I am skeptical about #1, as it didn't seem to really change much vis a vis folding and the consistency of the batter. As for #2, I'm still uncertain as to why this would make a difference, although I suppose it might if the oven thermometer that reads temperature (I have long since determined the actual temperature of my oven's thermostat, eliminating the necessity of relying upon my oven thermometer, which cannot be seen with the baking stone in place) is located below the baking stone, rather than above it. However, this is still problematic for me, because I did not use a baking stone when I made my cakes in my parents' oven in Montreal, and the cake collapsed even worse in their oven. (That oven was calibrated with a thermometer too)
As for #3, this seems to be the most likely possibility. The cake/pastry flour from Oak Manor is unbleached, which would be a bad thing vis a vis cake flour, which needs to be as low-protein as possible. (Those idiot organic people are so obsessed with eliminating "chemicals" that they don't stop to consider the benefits of some of them vis a vis actual quality! Of course, they don't bother including an actual protein amount on the package, so who really knows?) The Swan's Down is about 7% gluten according to their website, which seems slightly high for a true cake flour. Unfortunately, White Roses does not publish their gluten amount either online or on the package, so God only knows what it is. My hope is that as the cheaper brand, they throw in as many "chemicals" as possible and consequently yield a much lower gluten cake flour. My tentative conclusion would be that this is indeed the case, and as a result, their flour yielded better results. Either way, I'm using White Roses from now on, and to hell with Oak Manor and their pretentious organic crap. (Their bread flour sucks too, FYI)
Anyone have any alternate explanations?