My understanding is that the US paid to prevent the bail out of RBS in the same way that the UK stepped in to bail out Irish banks. The concern was not the technical or regulatory responsibility but the potential domino effect of those banks failing on domestic banks.
Of course, as with all these things, no one actually knows whether it would be permitted by the EU or the Bank of England for Scottish banks to be regulated and underwritten by the BoE, even in a currency union; to be headquartered in England but managed in Scotland outside of a currency union or whatever other combination might be proposed. I don't know, you don't know, Scottish Financial Enterprise doesn't know, Robert Peston doesn't know. It really is no good asserting that it is straightforward just because you wish it were so.
Nelly, what are your precedents? What happens in any other industry is largely irrelevant because they aren't governed by their own EU directives that require (a) banks to be headquartered and regulated in the country in which they do the bulk of their business and (b) for each country to have its own financial regulator.
It may well be very possible but for it to work you need:
1. the Bank of England to agree to underwrite a bank only notionally HQd in England but substantively operated from another country
2. the FCA to agree to regulate such a bank when it's main functions are in another country
3. the EU to agree to ignore its own directives just for Scotland.
They might all do that but I'm at a bit of a loss to understand why they would.