I honestly don't think there's a whole lot we can do at this time. Any kind of physical, martial, or magical attack on Valerian or his consorts would only result in our being Marched, whether individually or en masse.
Our best option there, I think, is to try to work within the system, very carefully. Try to find out whether Valerian has any steadfast allies or if any allies he has are simply people that he's bullied into allying with him.
As far as moving the Tribunal and keeping it entirely on the mundane side – I'm thinking that, while it may be a good idea in principal, wouldn't work so well in the long run. I believe that it's more or less common knowledge within the Tribunal that we have a Regio, and that most of our covenant is in said regio. That being said, I think it would work best if we hold the Tribunal within the regio, but try to confine everyone and everything to where we want them to be. Maybe downplay the sheer awesomeness of the place, give everyone just enough info about it to satisfy their curiosity and make them less likely to go exploring. And perhaps Creo a few corpses just beyond the Out of Bounds line, just to throw a scare into anybody who thinks that a nice romantic stroll would be a good idea. (We totally need to create a Dutchman named Jason, just to put him in charge of keeping people corralled).
In a related vein, I think it would be good for us, or our Legions of Faithful Minions, to explore our regio as thoroughly possible, and to find a bunch of exits/entrances. Not because I'm concerned about the "swiss cheesiness" of the regio (JL's satisfied me on that account, and boy, that is a phrase I am never repeating without proper context), but because I think it would be handy to know how to get out of it if worse comes to worse.
Re: Seeking out the Diedne. All I know is that, some time ago, they came passing through our upper regio (where the Blacksmith lives, and where Iosephus went when he disappeared), and have dispersed throughout Magic. The Blacksmith indicated that they were still in Magic somewhere, but didn't say where. ((Did Fiona get an idea of how powerful the Aura was there? Was it actually the Magic Realm, i.e. M-10, or just close?))
If, as we now know, the Diedne were basically framed in the Guernicus's power-game to keep the Flambeau and Tremere in line, then we may have a dickens of a time (1) convincing them that we're not out to kill them, and (2) working with them on a way to rehabilitate them (and I mean that in the canon law sense).
I'm torn about Laetitia pretending to be Guernicus's patsy so they don't kill her. On the one hand, it might be good to have a mole. On the other hand, I think she's outlived her usefulness to them, especially if they were planning on killing her in their ritual to destroy Mons Electi. It's possible that they will allow her to stay in Guernicus and just give her the shit details. Right now, though, I don't see them wanting to let her live, as she knows too much about what they're really up to, but I don't think they can act overtly since they don't know how much she's told us. I can, however, see them, grabbing her, letting her have the child, killing her, and telling the world that she died in childbirth. Okay, now that I've thought about it, I'm not torn, I'm purt much agin' it.
I just had a really out-there idea. What if we pulled a Xenophon and gave Laetitia and Glaukopis a new identity? If nobody's seen her since Apollodorus was killed, we can play off like she was killed at the same time as Apollodorus, or died from wounds she sustained in the attack. Fiona has Gift of the New Body that she can use and/or teach someone; once someone has learned it, they can then invent a Momentary Ritual version to make the change permanent. She can be ex Misc, nobody would bother verifying because those damn hedgies are such sloppy record-keepers that it's a miracle anybody knows who's in the House anyway.
(p.s. Thank Gygax for OpenOffice auto-restore after a crash)