As someone living in the rest of the UK, I bitterly resent the possibility of a part of the UK being on the other side of a customs barrier of any sort. Equally, some residents of NI might resent it the other way around.
(I accept the pragmatic management of the channel tunnel terminals because that was done for mutual benefit and by proper agreement.)
Yes Oyster, that is exactly the conundrum,which the UK has generated. The majority of NI trade is within its locality,and that means the hinterlands of Louth,Cavan, Monaghan Donegal, as well as into neighbouring NI counties Armagh,Antrim Derry. Then the finished goods,have the majority exported via Larne and Dublin to mainland UK. But in the calculation as presented by the DUP, they ignore the internal logistics and choose only to flag up the bottom line.
Reverting back to the original position, if the EU and UK accept a backstop as applying to NI only, then the problem becomes simply one for the UK to solve internally.
1. They can jettison NI and enforce their intended different customs arrangements for the remainder of the UK ..The EU will live with that...It would not be very principled, and I am not advocating it,but it would be pragmatic.
2. They can actively research and develop technological solutions and when one which is suitable emerges agree with the EU , allow the customs divergence to occur. The British press choose to make the erroneous assumption that the EU will never agree..I disagree.
3. They can construct new arrangements with the NI adminstration,which are agreeable to a majority in NI and which are aligned with the aims of the limited backstop... that's called politics
4. They can choose to remain with the UK in close customs alignment with the EU, so that these barriers do not exist
In the absence of one or other of these a disorderly no deal exit will result.