Referendums in the UK are not binding, but, the tradition of democracy requires that the results are observed. All matters proceed by agreement. Furthermore there is an overriding principle of governance by consent.
Constitutional matters are, by tradition reserved to the Westminster Parliament. Accordingly, if the Scottish Parliament were to pass a referendum bill, they would be exceeding their powers. In reality the parliament would proceed by consensus, and only pass a referendum bill with the consent of Westminster.
What if Westminster did not consent? Then there is a major constitutional crisis. Holyrood could try to hold a referendum, it would be chaotic. If the referendum passed a majority in favour of leaving the UK, then Westminster could prevent it by force. However, both parliaments know that this would be a disaster for the whole of Britain. Britain is a mature democracy, we just don't do that.
So, while Westminster has the constitutional basis for blocking a Scottish referendum, and holds financial and military dominance over Scotland. In real politics it wouldn't block a clearly expressed will to hold a referendum, nor a clear vote to leave.
As for your second question, you should split it off, I suspect it has been asked before. It would be possible for an Independent Scotland to rejoin the EU, but it would be a matter of negotiation between the EU and an independent Scottish government.