EDIT: I wrote this before seeing the latest chapter. Damn you, my inability to write short, concise paragraphs! You've screwed me again!
I mean, for all that she is constantly criticized, Henrietta is doing somewhat well in her plots.
Haha no. What Henry is doing is commonly called "giving them enough rope to hang themselves with". It clear she has some idea as to what her end goals are BUT, she is entirely reliant on Gallia doing the heavy lifting in the meantime. Because that's really the only way she could justify breaking off the marriage to the Germanian emperor. That aside, whereas Henry's plots are built on a solid foundation, Henrietta's are at least partially being played tongue-in-cheek. It might not seem like much now, but all of her plans are quite precarious and have way too many ways to fail.
A lot of them hinge on other people acting a certain way, namely Gallia, and by that extent, Henry, and it doesn't take David Xanatos to tell you that kinda plot only works out in the movies. I mean, lets look at it. The only way Tristain is going to get the manpower it needs to take Albion is with either Gallian or Germanian force of arms. A small country like that is never going to be able to raise an army to take back the country, much less police it and repair it's infrastructure. It's pretty clear from the beginning that she's almost totally reliant on foreign military power to boost her grand ambition of putting Wales on the throne, and then slipping in and usurping the glory in the aftermath. I mean, hey, it worked once right? Despite the notable loss of Gallian troops Henrietta just slid in and claimed it as "her" victory, and I bet that's what she was expecting from the ground up.
That said, when word gets out about what she did I don't think people will look too kindly on her, or too harshly on Henry for pulling out. It's one thing to throw away commoners. Nobles are expected to do that. It's another to try to claim a victory when it's clear you didn't fight. It was made clear that this was Henry's show. Usurping that town in the name of Tristain was like spitting in his face, and is indisputably dishonorable. In fact, in the old days, this would have been grounds for Gallia to turn right around and burn Tristain to the ground, and nobody would stop them, especially since it's becoming increasingly clear Henrietta is looking for an excuse to break things off with Germania. So yeah, as far as the good Princess is concerned, this is less high-speed gambit chess and more like a ten gambit pileup, and she doesn't even notice the noose.
Wouldn't 'reasserting the Albionian Monarchy' be enough of a casus belli? ...Would Wales agree to being a vassal kingdom to Tristain?
Eh, you're assuming Tristain is able to win Albion in the first place. It's been mentioned several times that a Royal seeking outside help to fix their problems isn't just a sign of weakness but also a sign that they're unfit to rule. Technically speaking Wales lost his right to the crown when he "fled", and whatever the circumstances were, that's what it looks like, especially if he shows up with a Tristan war host. Gallia, in a lot of ways, is in the clear here. They have the power to take the country and Wales doesn't have the power to make them give it back. At best the case could be made to have him come on as a figurehead ruler under a firm Gallian vassalage that basically strips the crown of Albion of all it's power, assuming they bothered with that at all.
As it is without Gallia or Germania doing the heavy lifting Tristain has no real hope of victory. If Henrietta uses the elf, she risks exposing herself to claims of heresy. What's worse, is that because the crown of Albion has been known to openly comport with elves in the past, AKA the previous king's fling with an elf that resulted in Tiffania, Henry could well claim that Wales is corrupted and spread that corruption to Henrietta, and by that I mean either willingly comporting with the enemy of all Brimir's faithful or being controlled into doing so. It wouldn't take much to start a purge, and if Henry plies it right he could even get the Pope to go along with it, since all that guy cares about is raising his army for his holy war against the elves. Hell, it wouldn't be hard. All Henry'd need to do is promise to raise troops and weapons from Albion and Tristain to create a mighty war host. He wouldn't even need to commit to a Crusade, just imply it, and then of course shank Popesondick in the back when it becomes politically convenient. He was planning on doing it anyway, so hell, why not?