Thank you so much for all the work you've put into this, Mattie! I really appreciate learning about the science of baking.
I'm curious, what are your thoughts about "Nina's Game Changing Vegan Butter with Aquafaba"? Have you compared this recipe to hers? From what I gather, the difference will have something to do with warm-temperature stability and flavor profile? I'm really looking for an ideal butter for laminated pastry.
Also, have you tried using a flavored coconut oil such as Nutiva's Organic Buttery Coconut Oil as a base for your butter? It appears to be a refined coconut oil, with the addition of "vegan buttery flavor". According to their website, "Our vegan buttery flavor in our Buttery Coconut Oil is made from pure certified organic non-GMO plants including sunflower, coconut, and mint."
As a side note, Miyoko's also produces a (cultured) vegan butter. I'm curious if you've tried it? Her recipe for home-made butter was published in a review, and includes some interesting notes about a "really hard butter" variation to use for pastry. You can find it here: http://anunrefinedvegan.com/2015/07/27/the-homemade-vegan-pantry/