Excellent comments folks.
Thanks for pointing out the comments in MoF, I'll look that up. Does this reasoning hold up? If a wish can pull it off, and wish can duplicate any 8th level spell, a spell tailored to create a dead magic zone could be lower than 9th level. Limited by the fact that a AMF is 10 min/level. So around 8th, with a hefty XP/material component price, and probably an important arcane focus. Sound reasonable?
And yup, this campaign is based on getting into it with all of Faerun's heavy hitters. The party are Thayans, and the goal is to kill the Simbul. Naturally, that means you've got to worry about Elminster, and probably the rest of the Seven Sisters. Then you start looking at offing all of Mystra's Chosen, which means you need to contend with Mystra herself. So they're looking to Shar, and they want to hit Mystra directly. Hence busting up the Weave.
But it definitely has to be done quietly, and preferably far from home. It'll likely start in Rashamen, at certain sites sacred to the Hathran, hopefully paving the way for a full blown Thayan occupation and massacre.
The wizard of the group is soon to join the Red Wizards, as the group has recently had a stunning success which will greatly profit the wizard's master. His master is always looking to place former apprentices well, and promoting the PC to the Red will fit her plans perfectly. At one point or another this group is likely to tangle with everyone in the Realms, and I fully expect they will join the ranks of the most hated and feared powers on the planet. Enemies and adventures won't be hard to come by.
Again, thanks for the pointers, I'll take them all into consideration (except the Chronomage one, I hate time travel

).
PS