I think this approach would fit better than the other one you suggested, but we’ll see.
Maybe! 
I don’t know how Praetorian the relationships are going to feel, but I’m sure there will be G5 opportunities for the MC to violently betray the major faction you’re allied with (if any).
There’s a bit of dialogue I quoted earlier from Cerlota that may be germane here:
“Also, it was said in the Lykeion that Kleitos holds the lives of the Nine in his hand at all times.”
“Does Sarcifer not disprove that?”
“Perhaps. Or perhaps it is just that Sarcifer is the only one to have found a way to break free.”
I’ve got no comment on whether there will be any possibility for other Ennearchs to betray Kleitos in G3/G4.
They’d make a hell of a couple. We’ll see.
Marriage alliances do solve some problems, but “my lover abandoned me because he thought we were going to die, the bastard,” is not one of them. Bringing Zvad back into the MC’s fold will be easier than bringing him back into Elery’s.
You aren’t going to have power to appoint anybody governor of Wiendrj in G3 or G4. For you to be the one calling the shots, the old order (not just Karagond, but indigenous Wiendish) needs to have collapsed or been subordinated, and you need to be the strongest actor in its place. If you take an imperial path in G5, that will be thinkable, but not before. And I’ve no comment on whether the first Wiend you met will turn out to be your best candidate for the role.
Well, yes and no? It’s fairly easy to imagine a G5 outcome where Avezia isn’t part of either Erezza or Shayard (or what’s left of them). Fragmented states and ungoverned areas all across the continent is clearly on the table. That doesn’t mean that in that scenario Avezia would consider itself “jointly owned” by anybody, though. An indepdenent Aveche would probably elevate the Avezchene dialect over either Erezziano or Shayarin and would have leadership based on local power dynamics, rather than an artificial balance of representatives in the name of the two former Karagond archonties to its east and west.
If your goal is eventually to reign over Shayard as a unified country, “Earlund” is not I think going to be a wise concept for you to promote. A major power-shift to a network of previously marginalized Westriding and Rimmer nobles (i.e. “Earlunders”) might well spark a civil war with the currently privileged Southriding and Coast bunch; you wouldn’t strengthen your side by trying to frame that war as an “invasion” of half the country by your other half.
Rather, “Earlund” nationalism might come in handy as a way of consolidating power in an area more manageable than the whole of Shayard – or as a divide-and-rule approach if you want the Laconnier movement to collapse on your path to a continent-wide empire that has smaller provinces than the Hegemony’s archonties.
The de Syrnon bloodline is a Shayardene one, i.e. from Grand Shayard. The idea of “Earlund” is of an older, culturally distinct area that was subjugated by Shayardene conquest. To cast yourself as both the heir of the de Syrnons and the champion of Earlund would be a really, really tricky balance – “I’m the One True King! Because my ancestors conquered and imposed their language and culture on this suppressed nation in whose name and honor I shall now begin to fight!” and inevitably you’d lose a lot of Laconniers (especially but not exclusively from the Coast and Southriding) if you tried.
You’re right that it would take a mix of CHA and INT work. I think there would be multiple pathways, any of which would need to involve NPCs – this would be too big a task to undertake alone.
We’ll see!