Tara and Strombran have done the exact same things during the Caergoth/Suville war. Both sides continue to complain, but very little is done about it. For your reasoning, CE could just as easily attack Strombran as Tara is an older ally than Strombran. Funny thing is, the federation is weird. There is no love between Tara and Strombran at all, but still enough fear for CE so it seems.
In the meantime, the Riverholm change should normally mean that the south goes back to peace again, because Caergoth can't break their alliance with Tara, as it means suicide, how fun!
Also if your Senate blocks all initiatives, one can still secceed as I did in Sirion.
Nearly all the dukes are Traditionalists, if one person secedes the rest will beat that duchy back into submission.
