The Northern Confederation while not a political entity Bilgewater would entice into aligning without provocation by it, does seem like a very good way to make Ionia angry and they already hate us for some reason. The one bad thing is that the Northern Confederation is in the north save for Mag Mell which could an easy target for military strikes, cutting off any supplies we get from there.
Centralizing the government will lead to less nationalism or just cause the Atlantean and Marai nationalists to revolt against Bilgewater. I don't see any good coming from centralizing so I think we should just keep Bilgewater decentralized, barring we got flooded by the aquatic dwellers.
I've said this many times but we should definitely not ally with Zaun. They're going to pollute our oceans and corrupt our officials, and not in a way that they already are corrupt. The only thing I think Zaun is good for is helping us make our dinosaur cyborg army/amusement park. We should ally with Demacia, Piltover, the Freljord, Shurima, and/or Bandle City instead.
As for Jarvan III, Jarvan IV is a giant monster hunting badass who doesn't have time to set aside to ruling a kingdom. Jarvan III has at least another good decade in him before he turns the throne over to J4, but Jarvan IV shouldn't rule until need it be.
The Demacian sanctions are merely political treaties without any regard to a counter claim of some ancient bill that excuses a nation from other enactions that contradict the terms of willful breaking of alliances, considering Demacia did save all of Valoran and all that. The sanctions should be lifted immediately so Demacia can go back to smiting evil as effectively as possible.