So Dies Irae mentions the 7 hermetic sleepers, and that of the 14 founders, only seven are confirmed dead: Flambeau, Guenicus, Jerbiton, Mercere, Tremere, and Trianoma.
This strongly implies that if the seven hermetic sleepers return, one of them might be Diedne herself.
Is she recognizable enough that the return of the seven sleepers might immediately erupt into a firefight?
Or alternately, how do you think the order would react to Diedne returning, especially in that context?
I'm pretty sure Verditius was killed in a feud with Corsicans?
Given the information purge in the aftermath of the Schism War, I doubt Diedne the Founder would be immediately recognizable. Portrayal of Diedne is forbidden and is among the things that has been purged (see Against the Dark "A Portrait of Diedne").
Not that it would matter, as Magi could deduce who is who quickly. Here are 6 figures matching the descriptions of the Founders whose final fate is unknown:
"Bonisagus, Criamon, Birna? (Its a bear), Tytalus, Merinita and Pralix!... Than, wait, who is that last one-Oh shit..."
How the wider Order might react would depend on how precisely you choose to weave the Seven Sleepers into the story.
Did they knowingly / willingly accept their role as the Sleepers, or assembled by a third party?
Were the Sleepers aware of what has transpired in their absence?
If Diedne has returned, are the survivors of the Schism War / her resurgent House with her?
Given the context, personally I think the odds favor Diedne. If the Sleepers accepted their role and purpose, then saving the world is a priority. If not? I don't imagine any of them, barring Tytalus and Pralix being very happy at what has transpired. Of these 6, Diedne only had a poor relationship with Pralix, and a good one with Bonisagus and Merinita. Tytalus is Tytalus.
I imagine House Tremere would be very difficult to reign-in, end of the world or not. Centuries of bad blood and paranoia are hard to overcome.