You make it sound like you could make a difference even if you wanted to 
Not all aid needs to be military. And honestly, grouped together, the southern realms sure could grant Caerwyn comparable assistance than Astrum gets from its buds. The south is distracted and somewhat divided, not impotent.
The king going against the express wishes of his entire council? Sounds like internal strife to me.
I wouldn't say so. Disagreeing on a single act which he thought to be innocent but which the lords correctly predicted to be disastrous isn't strife. It wasn't "we hate you and want to replace you, you tyrannical son of a dog!", but rather "You shouldn't have made Cato a duke, you twit". The lords didn't envision any scenario to replace him.
Yes, Storm's Keep had level 7 walls, and Thulsoma with 20 nobles could field 8kCS on the back of a total realm income that probably never exceeded 500 gold a week on a good week, and probably averaged closer to less than 250, especially as they lost their outer regions. As Indirik pointed out, we did not all show up at once to kick them down with overwhelming force. The war escalated gradually as it became clear that Thulsoma was not your average one-region realm.
I doubt they could still field such a force after the loophole was closed. Circumstances may nuance it, but to outside observers who couldn't care for nuances, that was pretty much the equivalent of 100 regions going to war against one or two. Patience would have served SA's PR better than acting as a bloc.
Any realm that says such things to/about such a massive behemoth should expect to get squashed. (Honestly, I'm frequently surprised Riombara didn't get squashed long ago for the things I said about Enweil—and they weren't a tenth as bad as what Thulsoma was saying 
)
You can thank invasions for that. The western coalition saved you in the first round, and all of Enweil's progress against you had to be restarted, but by the time the invasion started we were back in business deep in DoA's turf, having some nice looting times. With hindsight, I wouldn't have pulled the army back and would have continued looting the peninsula dogs. After all, you guys only took advantage of the retreat to ambush our stragglers, turning what would have been a glorious Enweilian victory into a useless and frustrating defeat. Should have known that the start of the invasion didn't matter as much to you.
As for before I had any political pull in Enweil... well, what can I say. People were too passive when I arrived.
