The Scottish gvt called the last referendum and the UK gvt accepted it with provisos. The precedent is set now. Even Ruth Davidson the leader of the Scottish Tories conceded last night on the leaders debate that the Conservatives would not try to block another referendum should the Scottish gvt hold one.

It is true that there is a hardened minority within Scotland who personally wish to never see another vote but polls show they are a minority. There may be one at some point but it isn't an issue for this election. Sturgeon has laid down how she views one could be had again. The SNP would need to stand at the Scottish election with their manifesto stating they would hold a vote and they would then need to win a majority at Holyrood. The days of "never mind what the Scottish electorate decides we'll override that with the opinion down south" are gone!

As to EU withdrawal then I think the idea of each part of the UK having a veto is a bit of a non starter which just wouldn't be accepted in England. However the legal process as far as the devolved parts of the UK being dragged out of the EU against their wishes is not so straight forward as you seem to think - especially if there is still an SNP administration in Edinburgh. It could well cause a constitutional crisis with another Scottish referendum being a possibility.

http://www.centreonconstitutionalchange.ac.uk/blog/british-withdrawal-eu-existential-threat-united-kingdom