One Through Ten

Tell her that manipulating the Will is a) probably impossible for you and the siblings and b) would count as tampering and blame would pretty squarely fall on the extortioner, excluding her from her share of the inheritance.

Let her know the proclamation of the Lawyer's death would be a huge headache, but that it won't throw the Kingdom into war+instability as she seems to be hankering for (wow, treasonous) as your master appears to have organised for this eventuality. Don't tell her any more about Stumpus (including how little you know about the guy) unless she offers information. Y'know, what with all these threats to the Will's safe execution their personal details (whereabouts, motivations, extancy) being a matter of utmost secrecy.

You'll have to hang up on the High Mage and address those other Lords, but she's welcome to call again if she can offer any assistance in regards to who killed the Lawyer with those rad divination+scrying magics of hers. Picking up a share of one Lord's lost inheritance has to be a better gain for Vera than whatever pieces she's hoping to pick up after the whole place has been ravaged by war, right?

don't play the bolded angle. play the italicized one. we actually have the leverage here and she doesn't even realize it. the only reason she'd be trying this underhanded stuff is if she believes she has more to gain this way than in the ensuing instability. otherwise she would have gone ahead and run her mouth. "go ahead and blab," we say. "have fun with your empire of ashes." in a war of this scale, even the winner will lose.

as a back-up plan, is there some way we can cast doubt on her testimony with the others?