Because you looked at the polls and the betting markets and you thought you knew better. Once again, you were wrong. Remember a guy called Kerry?
It is not the Bank's debt; it is the UK government's. You can read the DMO contracts if you like. And the debt does not have a monetary purpose: it is issued to finance government spending.
You're right, sterlingisation can't be blocked, but full-on currency union certainly can. I don't think anyone in the Yes camp gets that no-one in Westminster is interested in entering into a currency union - not with Europe, and not with Scotland either. What's in it for them?
You make the assumption that the planet will not transition to a primarily renewables energy mix. Given that a number of renewable sources are already competitive with fossil fuels and investment at scale is only beginning in this sector, I would suggest that is a hell of an assumption to hang your country's prosperity on.
So, the inrush of global partners wouldn't happen. More to the point, why would they rush to jump into bed with a government that has already stood up and said it is seriously considering reneging on UK debt? Which, by the way, is not the norm for newly independent countries and would be remembered by the markets. If they lent at all, they would certainly require paying for it. You really want to pay Greek interest rates on your government debt?
Think Venezuela, not Norway.