You are quite correct, NI is de jure staying in the UK customs union, however they will de facto be in the EU customs union and regulatorily aligned with all European laws.You seem to be living in a parallel universe...
NI are not staying in the EU... they will leave alongside Wales, Scotland and England. You might have confused yourself with the transition arrangements that have been constructed in such a way as to allow the EU to agree to remove the 'backstop'.
I have never said that democracy is irrelevant in Scotland... I said that the referendum was for the whole of the UK and that individual areas cannot elect for different treatment. (read back to my example of St Albans)
It would appear that you'd like to have a Scottish independence referendum every year until you get the result that you personally agree with. That's an odd kind of democracy !
It would be highly beneficial for the Scottish people, if the Scottish government used their authority to properly govern Scotland (in areas like education and health) rather than pursue the sole issue of a 'neverendum' ! Scotland have vast devolved powers, but have chosen to use very few of them.
This is why the DUP are concerned about an border in the Irish Sea.