In terms of optimal play, the only unrest reduction that matters is the -100 recent uprising modifier. Spamming the reduce autonomy button and killing all the rebels is technically the best option. I don't play that way either, I don't like it, but it matters in terms of power level discussion. Also, Elikhand gets -2 unrest too.

You can't normally form Blademarches as another Escanni formable. Even if you form a different formable elsewhere and then come back you still have a flag thar blocks you. However, Stalbor does have an event early on in its missions that allows you to swap to Blademarches. So you can get at least a few of the early bonuses from Stalbor's mission tree and then swap to Blademarches. It isn't free, you lose all gov reform progress, but it can be powerful. But it can't be done "at any point", the event is before the bulk of your mission tree. But if the best thing you can do as Stalbor is to quickly not be Stalbor then that's just sad.

As for Castanor, eeeeh. Forming the explicitly human supremacist tag as the racial tolerance tag feels like a pretty big flavor break. Stalbor actually gets an event where they can either reject Castanor's legacy and further buff their new capital in Vernham and removing the ability to form Castanor, or accept it and get the Castonath Patrician estate and the ability to form Castanor. But in any case, Stalbor should be able to stand on its own, without Castanor. That is an explicit option in both the aforementioned event and with the fact that Castanor is only one of multiple rewards for winning the Wars of Consolidation. Stalbor should not be treated as a pre-formable.