The Highland clearances OTL did a very good job of decimating Scots Gaelic to the point where to find majority Gaelic speaking areas now requires going to the Outer Hebrides and despite quite large efforts to spread it, the number of speakers is actually decreasing.
I precised, but I began to think nobody cares to remember, "
before *nationalism".
You're making two wrongs assumptions here.
1) Highland clearances themselves didn't broke Scottish national identity, it's still existing. It weakened the social structure of northern Scottish society, allowing a better hold for later national movements British or Scottish.
2) Language =/= nation. You have maybe 5% of Irish Gaelic regular speakers in Eire, the rest of the country being largely english-speakers, and yet they're not British or English nationals.
If we're assuming that Scotland has been unified with England in such a manner that there isn't 'Scots Law' or 'the Scottish church' or a Scotland defined any differently from one of the regions of England, then I would think it quite possible for Scots Gaelic to decline to at least the same extent and, more significantly, for it to become detached from Scottish regional or national identity.
Scots law or Scottish church are more, in my opinion, consequences of a distinct identity (even if a dialectal relation exists) than its cause. The distinction between Northern England and southern Scotland existed before them, and while the Scots law certainly strengthened it, it's posterior to Scotland existence.
Again, that's the main difference between Cornwall, Yorkshire and Scotland : while you had local elites and dynasties that were the recipient of a local identity (there's other referential, but it's a main one) there was a large conscience to be part of a larger social continuum. Scotland, on the other hand, was its own continuum in this regard (and actually had strong local identities of its own)
You know better than I do that there were significant Occitan (or at least significantly different in culture to Paris) dynasties ruling large parts of the south of France practically independently, and that most of the county was a patchwork of duchies and so forth ruled with separate laws and by long lineages of dukes right up until the revolution, yet no-one speaks of the Bourbon nation thesedays.
I don't get the point here.
I'm not really sure to understand what you want to say : are you arguing that nowadays, aka after nationalist movements, the national identity in Western Europe isn't based on dynastic reference? Yes.
But I precised, as said above, before *nationalism rise, that put national identity to others referential. See above and please, people, would you care to read all this post? It's not like it's that long.
I'd argue that if you've got some sort of union and the separate Scottish Parliament, legal system etc. has been abolished, then it's very likely that the population would, by the present era, view it in the same way that you get the general internal rivalries between different regions.[...] But significantly these are all viewed as regional differences rather than national ones.
You had OTL such union between Scotland and the rest of UK (even if it didn't destroyed its entire administrative particularities) and yet it didn't broke the Scottish national identity.
Again, doing such can be really useful as destroying social recipient of identity allows to provoke national movement and, if you manage to kill in infancy scottish national movement, you can replace it by something else.
An ATL exemple : Commonwealth living on, managing to have the upper hand on Scotland, systematically destroying Scottish social and political infrastructure to replace it with is own (if you want an OTL equivalent, French departementalisation and official/imposed use of french in all public and educational sphere), and being successful at this. You'll end with a British identity overlapping a Scottish local one, without too much trouble from local Gaelic speakers.
Again, disappearance or maintain of a distinct language doesn't make a nation.