For the merger thing, well, a king can't unilaterally destroy his realm. Destroying one's realm takes time and (foreign) resources. A ruler can't just click a button to delete the realm. And I'm not against the mechanic that prevents a realm's last city to switch out (go fix that bug, btw
).
So if a ruler wants to destroy the realm in order to merge with another, you need to 1) declare war, 2) empty the capital of all militia and troops, and 3) have the other realm come take it over.
If the realm's nobles don't want to merge... it'll be hard to do so. The ruler has no power to sack militia, he needs the capital's lord or the general's support. And none of them have any power over the mobile forces. So every noble who opposes the fusion can recruit to the max and sit in the capital. With those lvl 5 walls (or greater), even without militia a mobile army is usually strong enough to repel an invader. And in the case of friendly takeovers, the assimilating realm usually isn't ready to accept heavy losses for the move. Not to mention that this gives plenty of time for dissenters to protest or rebel against the pro-merger government.
As for the recruitment fair idea, I don't really see it tied to the cited capital issues. Those are interesting mechanics idea, but it's on the whole mostly a tweak on the current system, where a capital on the border helps reduce the refit cycle but intentionally placing it there is against the rules.
