In a UK context, of course the SNP is a single issue party. They want Scotland to be independent. The idea that they would be making meaningful contributions to a UK coalition government is laughable. They would be working for their objective of Scottish independence, which would inevitably lead to the end of the government of which they were part. The SNP has no interest in a UK government which governs Scotland, they want independence and to be free of Westminster.
Absolutely, and as far as I can tell, they will do whatever they consider necessary to garner support to that end, regardless of any particular principles. They love presenting themselves as a liberal social democratic party, because that's the way the political wind is blowing in Scotland and those are the people they need to attract to the cause of independence, so they're trying to woo them. If it suddenly became apparent that being an authoritarian hard-right party would serve the quest for independence better, they'd adopt that position in a heartbeat.
I really can't trust a party so ideologically committed to something regardless of whether it's a good idea or not. I'm an Englishman who has lived in Scotland for nearly ten years, and instinctively I'm opposed to inserting a border between where I live now and where I was born, but I also really don't think the supposed advantages are worth the downsides - very much like Brexit. It feels like independence, which is permanent and will have very serious and potentially damaging effects for some time, is being presented as a solution to a temporary problem in the form of a bad government. Yes, Scotland didn't vote for Brexit, but we don't even know if it's going to happen yet. Going for independence now feels like burning your house down because there's a spider in the bath.