The current setup will really suck for the South... There's nowhere for Rio to go other than Enweil, which they could easily crush (and would bring them back to square 1: unable to do anything anymore). As for Enweil, there's nothing they can do, because anything they do risks pissing off Rio, which could easily crush them.
Rio could splinter, but it's so huge that I'd have a hard time seeing any secession being able to successfully fend off the rest of Riombara, lest it be a friendly secession.
Enweil can't really splinter. It would be devastating... We've had the Fheuv'n experience, and we saw how that totally backfired.
And even if one of Rio's western cities split, attacking Creasur (the closest possible target after Enweil) would be grueling, while it would take forever for the farthest cities. For anyone else, they'd need to spend weeks travelling to bypass two realms...
Which means there's no diplomacy possible. Either Rio bores itself with nobody nearby to bully, either it swiftly stomps Enweil and then bores itself with nobody nearby to bully. Even if Enweil managed to get anyone against Rio, there's nothing anyone could do to make the conflict interesting, because of how far back they'd be.
A Rines/Fronepu passage, over the newly formed sea, would at least open up possibilities for Rio to go North, or for the North to go to Rio. And the fact that it's over a new body of water could justify why a route never existed there historically (I'd rather more routes, but that's the only one that makes sense and is balanced, imo).
