@CirclesWithinCircles - you say a lot of very sensible points regarding the readiness of Scotland for independence and being a future member of EU.
I doubt that anyone who works in international trade would seriously propose Scotland joining the Single Market on the basis of Northern Ireland. That is a hellish arrangement which is not finished.
Honestly it would be very good if someone actually listed the criteria for EU membership now (and I don't mean the deal Britain had) and see where Scotland comes. It is nowhere near. Maybe in ten years if now and worked hard to bring its constitution, banking, customs agencies, taxes, public spending ratios, public finances, debts into good shape. That wouldn't come over night and there is nothing certain about any of it.
Not so much jam today, or jam tomorrow, but jam infinitely delayed until a set of criteria acceptable to the EU was met.
You also raise implicitly an important point about GB, which is that this is reserved to Westminster for trade purposes. I can see why the Alba party are pushing for it now. In a few years the alignment and patterns of trade will have changed away significantly from EU standards for Britain and that will not be easily reversed.