I'm thinking that Indoril warriors could be stirred to the point that they would rebel against the Imps and then the Redoran would have no choice but to follow since the conflict has been officially sanctioned by the Tribunal ie "Sotha Sil". I think that if Almalexia were dead and Vivec disagreed that her absence at least could be manipulated so that her followers would side with Sotha Sil given House Indoril's hatred of the Empire.
The only problem I see is if Vivec didn't want to fight the Imps for some reason and he tried to hold his warriors back in Vivec city itself.
I was thinking that in order to win the people over(in Mournhold) given that he has been blamed for the attack on Mournhold itself he could put it down to an experiment gone wrong and to make amends with his people he wants to give them something - freedom from the Imperials.
Thoughts?