The fact that the absorption of the weave suddenly starts faltering at a time when "the bomb" is required, somehow reinforces this idea. If Mystra created the process of weave absorption, it should be she that also breaks it down at the convenient time.
Except, she also explicitly sent Elminster to go and stabilize the orb. Which renders it completely stable for the rest of Gale's journey, no artifact suc required.
Thus, it seems like she's just making use of the situation as it has unfolded. Hence her also conveying the message to Gale to use the orb to wipe out the Absolute alongside this stabilization (Which is both evil from a mortal's perspective and also shows that she doesn't care too much about Gale's own wellbeing)
