I have no doubt that D'Tan is true to his word, and that Reman character of who's name I can't spell (so I wont even try) wants peace and equality for his people. I think they are both genuine about wanting peace with their neighbours, so for as long as they're both around, peace is what we will have.
The Tal Shiar still exists, though they might as well be thought of as their own faction now. Any Romulans that grow up on Mol'Rihan that would otherwise want the old ways can go join up with the Tal Shiar (no doubt the Tal Shiar would have operatives on Mol'Rihan looking for talent anyway).
They will no doubt eventually build up a military force again, though for the most part, that'll probably be to defend their borders. I can see them becoming allies or friends of the Federation, and its safe to say that (knowing by later centuries) at some point in the future they will join the Federation; just as the Klingons will.