"keep the £ and then join the EU"
Not quite. To join the EU requires joining the Euro - the UK had an exemption - and to join the Euro requires having a stable currency to establish an exchange rate. So Scotland would need to have its own currency to use in the transition period where it was independent from the UK and not yet an EU member. As I recall the option of choice was a Scottish pound pegged to the UK pound.
The thing with membership not being guaranteed was that any of the bigger nations can veto new members and they don't even need a good reason. As the other poster said, Spain was having edgier than normal times with the Catalans at the time and stated that they would veto any new nation that didn't gain independence through an official referendum authorised by their parent country - having no intention of ever giving one to Catalonia, of course. This prevented Scotland unilaterally declaring independence if the UK refused to give it. If Spain had vetoed after a successful referendum it would only have been to make a point to the Catalans and they would have been lightly negotiated into relenting by the rest of the bloc.
And yes, the EU did say that Scotland wouldn't be automatically grandfathered in. However, there isn't really a queue to join as such; there's just a set of requirements that you have to meet, which takes time. The most important of these is alignment of laws, which wouldn't have been an obstacle since as they were part of an existing EU member Scottish laws were already fully aligned. The process of gaining EU membership would therefore be quite swift. But Scotland would still have had to go through it, and that meant it would leave the EU when it left the UK. Hence the transition period I mentioned above.
Biting the hand that feeds IT
