By state, do you mean the institutions, the politicians, or the people?
if the European national states disentegrate even more of the functions of the national state will have to rise to the European Union.
No they don't "have to". I would prefer more would rise to Community level (and I also think that would stem the disintegration process, because it would make having a national state of less consequence); but there is nothing about the size of a national state that precludes it from having certain functions.
So as the structures of the nation state collapse
What you described is NOT the collapse of the structures of the national state. It is larger national states being replaced by smaller ones. As the process could be nonfinite, this can mean that the structures of the national state are inherently instable, but they have been already in the past.
While Europe faces few external threats, it will dissappear from the world stage as a player if it has no military power.
Migeru already flagged this, but worth flagging again: I am not interested in building an empire, and that's what military power is needed for.
By what right should a England sit on the Security council?
The same as Russia: main successor state holding nukes. (Though again, I'd prefer an EU permanent seat, or better yet, the abolishment of permanent seats at the UN SC.)
Will the UK allow Scotland to leave peacefully
Just the other day, someone posted a poll that a majority of both Englanders and Scots would approve. Unless the rabid wing of Tories took over, I don't think the government would decide to do something violent about it. But violence might came in other countries, especially those where drawing borders would be of greater difficulty.
In the long run, Europe might be better off with 75 states instead of 25
In the even longer run, those 75 might turn into 225, and then 675, at no smaller cost in lives.
And once tribal warfare ... is let loose, can it be put back into the bottle?
Yes/no. Yes, because no two historical events run alike, and trends get distorted run out, as could one learn from any past time when people spoke about a political trend sweeping the world. No, because it is already 'out of the bottle', of you look a bit more to the East. *Lunatic*, n. One whose delusions are out of fashion.
I'm not so sure about that, but hope you'd be correct. These sorts of polls about what the popular sentiment is don't take into account the underlying economic issues. Scottish independence is often portrayed as some sentimental yearning. From what I've read, some of the support from the English populace comes from a "cut them loose" mentality. One underlying narrative I've read in repeated articles is that Scotland is poor and dragging England's stats down.
This ignores the opinion of what we might call the elites -- those with something to lose. How would they react and what would they do to protect their interests?
Admittedly, I'm not all that up on current Scottish politics and haven't been there recently, but in my travels there and in talking with various people, there have been 3 very persistent themes over the years. The first is the poverty in Scotland. The other is the idea that Scotland has resources, but the profit goes elsewhere -- the most obvious being the North Sea oil mentioned here already. The third, and this is probably the most pervasive theme I've heard, cutting across regional and political lines, is land ownership.
It's not something I've heard much about outside of Scotland, so I'm not sure if that means it's common knowledge that doesn't need to be stated, or if it's not well-known, but it seems an overriding concern, especially in the Highlands. One of the figures that is bandied about is that over two-thirds of the land is owned by about 1200 private owners. It's difficult to even find out who owns some of the land in Scotland with some ownership issues going back to feudal times.
Given these conditions, I have difficulty believing that popular opinion matters very much when speculating about official reactions. If it's been this much of a fight even ascertaining who owns the land, I have difficulty believing that same land would be easily given over to any sort of independent governance or oversight. Maybe we can eventually make language a complete impediment to understanding. -Hobbes