I think if a referendum is held it's probably almost guaranteed that Yes will win. The problem is actually holding the referendum. Yeah they could just do it unilaterally, but they don't want to end up like Catalonia, so first they're trying to find or invent a legal basis for holding one without Westminster's approval.

and the EU wants access to those resources, primarily North Sea oil. I imagine the SNP has some kind of backdoor deal with the EU to have Scotland as an EU member as soon as an independence referendum is passed.

Ehh, the EU is about as rules obsessed as any other neolib institute. It would still take them years to actually join the EU. But I'm sure some trade agreements would be set up relatively quickly.