About whether it makes sense for Scotland to leave the UK, you are correct -- economically it does not make sense, but the same argument could be made about the UK leaving the EU (since 44% odd of the UK's trade is with the EU). Economics play a role in people's minds when deciding on a referendum, but emotions seem to play a larger role
As for Northern Ireland -- your point is also valid, it could go the other way with Ireland leaving the EU.
Alex Salmond is a has-been and he will always remain bitter about his defeat in the referendum and always angle for an excuse to have another one. He won’t find it though. If we left the EU, they’d have to present an even weaker economic case for leaving than the one that lost them the previous referendum. Scottish independence is a dead issue for the next 20 years.