Being a "Provisional Captain" just means she's a substitute Captain.
What she does is all well & good for the current situation, but a "Official Captain" will come in and take over Rios spot and handle the situation.
Until then, Seven will do just fine. And she has the backing of Admiral JLP, and I'm sure Admiral Janeway would back Seven as well.
Having two legendary former Captains, now Admirals; backing Seven is a guranteed way to have her actions stick.
And the fact that these new "Borg Cooperative" prevented a Galactic Scale Event from becoming a tragedy speaks volumes on their actions. It was a bit harsh and forceful, but they can learn tact and diplomacy with time and negotiation.