What you are talking about is about the heel angle that represents the better compromise between power (RM) and drag. The heel point were the sailboat will sail faster.
That's right, that's exactly what I was talking about.
If a given boat, say a Finot-designed boat, stays flatter when sailed optimally (best speed for course and conditions) than some other boat, then even if total area under the curve is the same between it and another boat, it has a significant "head start" so to speak and in practice will require more energy to capsize.
Not more energy from zero degrees, but more energy from an angle hopefully similar to what the boat was at before the gust or wave, as the flatter-sailing boat "has more of the curve left."
As you say, it's not irrelevant how flat the boat will tend to stay in the first place. The number for a given condition when the boat is sailed properly is not zero. (Well for some conditions it will be zero. But for example, it's unlikely to be zero at say 20 knots TWS upwind.)
And for the given condition it can differ between boats, sometimes by quite a bit.
Your other considerations you were talking about are on different points than energy required to capsize beyond optimal heel (as opposed to beyond flat, which I think isn't as useful as the difference has already been used up) and I agree with what you're saying.
To summarize really briefly, what I was saying is that while it's impossible to reduce stability to a single number, a small set of chosen numbers could give a reasonable picture. I was trying to give examples of a small set.
The "Q" value posted earlier is I think a nice example of a single number that can give a guideline, but as you said back at that time, not the whole picture.