Nice post @Electrifying. Very nicely put. I will however just mention some things in response.
Firstly, you're dead right on the distinction made between a unilateral declaration of indepedence (i.e. Kosovo and potentially Catalonia) and a bi/multilateral declaration that Scotland would be expected to represent, unless of course they go ahead with a referendum without the backing of Westminster.
There is also a large degree of goodwill towards Scotland within the EU, namely Germany. Mostly because they seem to 'get' the EU project, and because they are being taken out of the EUdespite voting largely for remain. There isn't as much antagonism towards Scotland from Spain as is commonly reported online and in the media. Yes, Spain no doubt prefers the UK to remain together, but I don't see why they would not support and have relations with an independent Scotland. Most of the langauge coming from Madrid is not anti-Scotland joining the EU, just a worry about the precedent that it would set.
However, in regards to the Spanish position on Scotland joining the EU. It's clear that Spain will force the issue that Scotland leaves when the UK does, regardless of any possible transitional deal. There's also a question as to where Scotland will be placed in relation to joinging the EU. I think that Spain would not rush any accession into the EU, and recently I've read that Spain will favour Scotland joining the 'back of the queue'.
So whilst Spain would view Scotland differently to Catalonia (despite the boost it would give to seccessionists), I would not expect Spain to favour any fast-tracking for Scotland, which would result in Scotland spending a fair amount of time outside of the UK and of the EU (perhaps EFTA membership could compensate this?).
Also, I think May will probably block an independence bid until after 2019. No way would she continue to block it once the UK has left the EU.
Edit: Only about 10 minutes after writing this post I noticed this on the Guardian:
A pretty good post on the position of the Spanish Government towards Scotland, and why it may be best that they put their indepedence bid on hold until Brexit. Giles Tremlett is a prettygood author and journalist, and I believe he's lived in Madrid for a while now.