I don't think the Gaians would let the Mollies put a base there. I don't think they'd be able to stop a concerted effort, either. The Mollies have a fleet. Gunboats, perhaps destroyers. They've multiple stations, wealth, and have fought the BAF to a standstill for...how long since the founders day revolt? Gaians are great. They're great because they're underdogs, not alphas.
And the Gaians going rabid? Again, they're not that big, and besides, right now, the Gaians are doing what? Laying in supplies for the winter? They can only go rabid for a short period of time without collapsing, same any group. They're not a huge group, either. And might I say, Bretonia is -already- fighting a four front war, against the Corsairs, the Kusari, the Mollies, and the Gaians... Cambridge is a front, Leeds is a front, Tau is a front, Dublin is a front...
I'm not at all convinced the Gaians going berserk is such a horrible thing. In the scale of things, the Gaians isn't a huge faction. Its not very big when compared to the Mollies, and its certainly not very big when compared to Bretonia.
Have the Gaians changed anything, ever? I'm sorry, Marburg, they're not so powerful.
As a bordersystem, I might suggest the 'borderworlds' of Gallia...and that a gate is more likely to open up trade than the BAF claiming the system. If you are suggesting Bretonia lose Orkney, simply because a new house is nearby...I'd suggest rather the opposite would happen. They'd grab it up, what once was a backwater world. This route, to Orkney, from Edinburgh, bypasses the Guard system which has so recently become one of the main conduits...
You ask why they would get the access without the buffer, like they're getting something great out of it. Buffers happen, or they don't. They're natural occurrences which don't make too much sense in the context of nations and Empires....and the two great Empires in this game? The don't have buffer zones between them. Why should this new place get any different treatment?