Yes. An avatar of Mystra was corrupted by Bane so there is precedent.
As to why Selune didn't intervene - I assume she did so in the form of granting her clerics the ability to fight back. We have to remember that when Selune descends to directly challenge Shar's followers Shar shows up soon after. And Selune is weaker than Shar, her power waxes and wanes while Shar has only gotten more powerful by absorbing other gods.
Of course Selune is usually backed by Mystra but I don't think Mystra wants to yet another direct confrontation between the sisters- the battle could tear apart Baldur's Gate.
Now I'm just making this up - I only have the silver-white of the magic, the fact that SH says the power is 'definitely' not Sharran in origin, the ambiguous language around 'the weapon' and the fact that SH is interested in the names of kids in the village to back up my theory.
And of course the guess that Larian wants all origin characters to be equally compelling. If Gale is a chosen who has a shadow mythallar in his chest - I wouldn't be surprised if Lae'zel was literally Vlaakith's daughter, that Wyll could become a grand duke of baldur's gate . . .
in fact, in datamine
there are voice lines of Shadowheart saying prey words about Selune.
. As for Laezel, I think it is very unlikely. the current Vlaakithi is a lich and known for not having any descends. otherwise, Laezel would just be the next Vlaakith queen.
