The geographical state of Wales being birthed now never was a unified Kingdom though it was very briefly conquered by Gwynedd (Northern principality in Wales), only for so much time as it took the English crown to realise and quell the 'rebellion'.
For GB there are about another quarter million people who are subjects of Her Majesty who remain in overseas territories.
If you're confused I suggest reading my first comment again...
I trust that you always refer to the Republic of China too, rather than Taiwan?
But the arguments at the time of the referendum didn't get stronger. The EU would require Scotland to join the Euro, join Schengen (means a border wall), and the primary argument for an independent Scotland was "we'll take all the oil money for ourselves". But now oil is cheap thanks to US fracking and the North Sea oil industry is going bust.
I think the future probably has more devolution in it. Which is fine.
We've also had almost 2 years of Westminster failing to deliver any of the promised devolution. We get these things are slow to happen but there's been pitiful progress towards them so far. Lots of people voted No on the basis of more devolution.
So while the arguments haven't changed the alternative to independence has for the worse. I'm honestly struggling to see a future without an independent Scotland tbh.
Hadrian's wall and miles of rural waste should already be sufficient for these purposes.
Seems like it'd be easier to get illegally directly to England, than go through Scotland...
As for NI - for the sake of peace that would be a more drawn out process.
Sinn Féin have called for an Ireland-wide referendum on unification because obviously they have. Even if you grant that they don't have the ulterior motive of being Irish Republicans and that they genuinely want Northern Ireland to remain part of the EU (and there's no reason both of these things can't be true), that's probably the only way to ensure that the six counties of Northern Ireland do remain part of the EU in some fashion. And, of course, if it did pass, you'd simply end up with a resurgence in sectarian violence as Unionist hardliners rebel.