Who says anything about giving up part of the Union? Though if you want to take the Scots off our hands then please by all means be our guest.
The principles of devolution long predate Brexit. But I'm amazed you'd be willing to consider a part of the UK having completely free and unconditional access to the Single Market, regardless of how trade talks end up. Seems like a wonderful backdoor for us to have as policing any customs between Great Britain and Northern Ireland if applicable would be our job not yours and we would look to make that as frictionless as possible so if our companies can simply transfer stock to NI then get free access to the Single Market without being part of it then win/win.
I'd think you'd be mad to agree to that but it seems to be your own idea. Are you guys crazy?