The backstop required for the WA is that there is no NI border.
The obvious solution is Norway+, i.e. the whole UK staying in the Single Market, plus a Customs Agreement
This is what Barnier has gently tried to recommend
and the whole UK could keep most of the advantages of the EU: trade, agencies, aviation etc
If the UK red lines - FOM, ECJ - rule this out, then the EU have said they would make a special case for NI alone
being in the Single Market under different terms to EU members
They could justify this to the WTO because:
NI has a small population, the GFA is an international treaty, the risk of the Troubles returning.
Plus no WTO member would be likely to object to just NI.
Scotland is a very different case legally & practically, because:
they had no recent civil war, no GFA registered at the UN, no land border with an E27 country, no likely majority in the next 30 years to unite with that E27 neighbour
Hurt feelings in Scotland don't change those 4 major difference to NI