Sure, but that split did not exist when the tribunal formed in 9C, and is thus irrelevant to historic formation.
Let me be clear that I thought we were discussing game world design, i.e. how the tribunals should form on the Hermetic timeline. If we discuss a subsequent merger in 13C, the question is of course very different.
Sure, and if the grand tribunal of 1228 decreed that there is one tribunal too many; now LL is disbanded and have to reform with Hibernia and Stonehenge, that would be the right consideration to make.
My point was that for LL to form in 9C, there would have to be a critical mass of covenants outside the Saxon and Gaelic spheres, at that time. Maybe there was, maybe there wasn't. A narrative can be designed to make the three tribunals as written plausible, but it takes some assumptions of Hermetic demography in the 9C. Looking at the map of 1220 does not tell us much.
One interesting aspect of LL is that there are only three canon covenants. Many more exist, but they are generally short-lived. This suggests that LL is only barely sustainable, easily failing to reach quorum if there is a dip in the number of short-lived covenants. Is that the saga theme? Fighting for survival? Was there more covenants in 9C, making the tribunal less dubious at that time?
That sounds like a drawing board design. The language groups are well-defined, and make easy tribunal borders. However, I very much doubt that 9C magi sat down to draw borders on a map as an intellectual exercise. I am sure, instead, that there were strong factions fighting to establish respective tribunal to represent their culture, whatever that was. Were the cultural links between p-celtic and q-celtic areas so strong in the 9C? Wales had been under Roman rule, and Welsh had split from Gallic eons earlier. Even if the Saxons never invaded, the East-wards links may well have stronger.
Your idea is very interesting though. A pan-Celtic tribunal would create maritime theme. Inter-covenant relations would require sea travel on a regular basis, and stories should reflect this. I have no idea if that would be historically plausible, but it would be a setting I would love to play. It takes quite some work though, to make enough details to make the setting easy to play.
The most important point, though, is that shifting tribunal borders is not a quick fix. It takes effort to make the new tribunals playable.