Sci
In 'Losing the peace' and ZSG it is directly established that the federation is internally unstable.
Sort-of. You're overstating it.
Losing the Peace established that the refugee crisis was threatening to create instability, but they
solved both the refugee crisis and the Alpha Centauri secession threat in that same novel. The only hint of internal instability in ZSG is a little bit of foreshadowing to PoD.
If Canada would be unstable enough that its lands were seriously contempleting secession, to the point that Quebec would secede,
Quebec has been seriously contemplating secession for
decades, to the point of there being several referendums on Quebec's continued membership in Canada.
About UK still surviving when the Thirteen Colonies or most of Ireland seceded:
You actually think that a federation composed of Earth and 5-10 other species counts as a federation still intact?
No. I think that a Federation composed of Earth and over
one hundred other worlds counts as a Federation still intact. Remember, the Federation had just added its 155th Member right before the Borg Invasion. Even if we assume that the Borg destroyed 30 Members, that still leaves 120 Members -- 119 after Andor's secession.
The Federation has lost Members before. Cait, notably, keeps seceding and rejoining. It's certainly fair to say that Andor's secession is a serious blow to the UFP, but that doesn't make it a fatal blow.
ETA:
Don't count Vulcan as 'in' in the long run just yet, Truth:
In Titan;'Taking wing', is is directly stated that only a small majority was against vulcan-romulan unification - meaning a large minority was all for it.
You're overstating your case. That a plurality opposes something does not mean a large minority is for it. It's entirely possible for the remainder of people to be divided between people who are for something and people who are apathetic or undecided. Also, you're forgetting a crucial element of any attempt to gauge public opinion: How
strongly do the polled individuals feel about the topic at hand? Someone can be "for" something yet never think of it as a strong enough priority to exert effort for it.
In 'RBoE', Spock's unification movement receives a major boost - now it's able to expand in the open.
Also in RBoE, Spock directly says that one of the ways he envisions vulcan-romulan reunification is the vulcans leaving the federation and uniting with the romulans- and he has no problem with such an outcome.
You are, again, over-stating your case. You are, after all, talking about the eminently logical Mister Spock, who can speak in perfect serenity about his own imminent death when discussing a given situation's potential outcomes. Spock, being the frank person that he is, simply answers the question of how Vulcan and Romulus might reunite by listing a number of possible scenarios. That he includes a one scenario -- that one of the ways in which Vulcan might reunite with Romulus is through secession from the Federation -- in such a way indicates anything about what he actually advocates about the future of Vulcan's status within the Federation.
But you know what
does tell us what he thinks about Vulcan membership in the Federation? The fact that, when he thinks the Romulans may pose a danger to their neighbors, he contacts the Federation President rather than the Vulcan First Minister.