So about them spoils:

The weave is a large piece of divine essence of mostly Selune with a smaller piece of shar. That is the origin of the goddess of magic. Shar of course knew this and eventually made her own weave known as shadow weave. Karsus is currently a rock and can even be a patron. It is possible that Gale is like a piece of Karsus but highly unlikely. The Goddess of magic has been known to go as far as protecting dracolich's as long as they make new magic. If what Gale says is the truth, highly unlikely the whole truth, Mystra rejected him for something more serious. If it was regular magic, Mystra could just cut it off or kill him. It's unlikely Gale outsmarted Mystra's way to stop mortals from using high magic. My guess is that what he has does not come from her but someone else. Possibly Shar, but it could be from anything divine or stronger. I also suspect that the tadpole nerfed his levels rather than Mystra so something does not add up.

Their is also a long list of mysteries including the following: why it needs to be maintained with weave in the first place, how did he seal it in his body, why necro damage, where did he find it if he has never been to the astral(party banter with laezel), why can't an Elf wizard just instantly know since elves see the weave differently, why isn't Gale being hunted by Mystra if she already knows what it is.

If Larian sticks with lore, Gale is quite the deceiver and not good at all. Approval from him could be him liking being put in a favorable position to manipulate.

Unlikely theory 64: ethereal version of shadow magic. I doubt Larian would allow the border ethereal or deep ethereal though. The void planes are more likely with all the tentacles Larian could want. Lol

Unlikely theory 65: Mystra is just sluuty and she got bored.