Donatra might have been friendly to the UFP, but I'm not sure she is in favor of unification with the Vulcans. Sure, she might make a bid for Romulan unification and at least try to make herself Praetor of a United Romulan Empire (URE?).
I doubt Donatra would be in favor of unification with Vulcan, but presumably the "unification" the other poster was referring to was a reunification of the Romulan nation.
If Donatra were to take power over all Romulans, though, she wouldn't downgrade herself to Praetor; she would keep herself as Empress. No one willingly goes from being a Monarch to being a mere head of government.