The issue is, while we may not be vulnerable to divination any more, the people that we convert will be. All the Lannisters would need to start asking is: "Which Houses plan on backing foreign threats?" Sure, that will ping on others, like Deep One, White Walker, Fey, or Fiendish, patsies, but it would still hit them as well.
Divination is extremely hard to get around. Write something down? That can be divined. Tell or convince someone who doesn't have Mindblank? That can be divined. Order troops to begin forming up for war? That can be divined, even if the troops are not sure where they'll be attacking.
Mindblank works great on the squad scale, when you're talking about a dozen actors at most. On the scale of nations, there's simply too many people who could break secrecy or be unprotected. It's also impossible for us to adequately cover everyone necessary in Mindblank; it would break the bank.
We can still ferment treason among the Westerosi, but it's not going to likely be any type of surprise for the Lannisters. They'll know and they'll be able to act and isolate our pawns with their own.
