Questions are good, that's how we all learn.
I think part of the confusion may be in "how many extra petals a double has to have to be classified as a double" .
The answer is 0. It's not how many extra petals, it's the whorl placement of petals beyond the petals of a single flower (or having petaloid stamens) that makes a double flower/Double Form. A Polymerous flower is a single flower even though it has more than the normal 3 sepals and 3 petals, they are in the first and second whorls.
Single Flower Whorl Structure
first whorl is the sepals
second whorl is the petals
third whorl is stamens
fourth whorl is pistil
A double flower changes the third whorl to petaloid stamens or more petal whorls. The first and second whorls don't change, meaning there will be 3 sepals and 3 petals. There will be 6 stamens. A hose-in-hose Double will have more petal whorls between the stamen and pistil whorls.
A hose-in-hose bud cross section.
I can see what you are saying about Avedon's image and that those two petals looked stacked or beyond the second whorl, I don't think they are. They look bunched up in the second whorl, for lack of a better term
, due to the size of the petals, folding and constraint of the perianth tube. Maybe "gathered at the top of the perianth tube" would be a better way of thinking of it, like trying to twist a large piece of cloth into a small tube.
Here's a simple 4x4 polymerous cross section showing what may look like "double petal stacking" or hose-in-hose inside the soon to open bud. When open this polymerous bloom could have had a layered look due to the left petal being out of a normal petal fold, normal being one edge on top of the neighboring petal and one edge behind the other neighboring petal. The left petal has both edges on top of the neighboring petals, but the base of the petal is still in the second whorl at the top of the perianth tube.
A 5x5 polymerous bud cross section showing the same out of the normal gathering of petals.
and the same bud cut just above the top of the perianth tube showing the actual second whorl placement of the petals in relation to whorl structure. I adjusted the image so hopefully you can see the parts better. Start with the outer sepal whorl, the sepal edges go to the center of petals. You can see the petals are all in the second whorl. Stamens are seen in the third whorl. Center is one very confused ovary.
If you look at Avedon's poly image, you can see the extras in the sepal, petal and stamen whorls - a Double would only have 6 stamens. It's a polymerous bloom. The only other option is a fused bloom.
A multi-form, having 2 or more recognized Forms, Polymerous and Double, has to have the floral structure of both, a poly and a double.
Poly def
https://daylilies.org/daylily-...
Double def
https://daylilies.org/daylily-...