Judging by the CC so far, I think the Sovereignty would be allied with the Supremacy under very specific rules. It'd be more like America and Russia teaming up against Germany in the 40s; they don't really like each other (because of the vampire thing; the Sovereignty don't trust Mercer) but they're forced to work together for the good of the world. At least, the good of the world from our point of view
The Sovereignty and the Covenant would be a more friendly alliance, I think.
There's really no reason for this since you won't know about the vampirism until they tell you. So really, unless your king has magic powers and can know what you know, he wont know
Now that you've mentioned diplomatic marriage, I think Alex might try do that. He might try get Jason, who is roughly '22'/ '23' to marry one of your daughters, or even Charlotte to marry.
Charlotte will never do that, but family turmoil is always fun. Alex rarely ever leaves his lands as he always has other things to do, legitimate things the people know about.
But overall..That shady Alex will manipulate you Epic...
I'd hardly call it manipulation, he's just hiding what he is because he knows he'd face discrimination. Hell, its almost like homosexuals coming out of the closet, and he is in the closet of vampirism.
That's how I see this vampirism thing, for Alex, as he hides it for the same reason. If someone gives me a bad rating because I mentioned the word 'homosexual' I am seriously gonna flip my pl*ps.