On immigration, for Scotland, at least, IMO we need a lot more of it (and I think the SNP agreed). Not only for the usual reasons given, like we need more working age people to support our increasing numbers of pensioners, but also because more people means bigger local markets for goods, services etc. The drift to London and the SE is because these areas have jobs and, for the most part, often a pretty good quality of life.
There is a virtuous circle of increasing population, leading to greater demands for good and services, leading to wage inflation, leading to more job opportunities, leading to more people etc etc. (I can't back any of this with statistics - sorry - but it is pretty self-evident really).
On the other hand, I can't see enough reasons to draw people to Scotland from the rest of the UK or overseas. In Glasgow there is some manufacturing, some decent universities, some financial services, but I can't see anything here which would match the vast numbers that are employed in the City, for example.
So, I think some major investment in industries and sectors which will deliver sustainable long term jobs, which will draw people to the cities, would make a major contribution to solving a number of issues.
And in terms of immigration itself, I think this should be partially devolved to Scotland, so that it could, for example, allow more economic migrants to work here (as is permitted in Canada and Australia, I believe). England might not like it but there could be ways of managing it, for example, social services and benefits would only be available to economic migrants in the city/council/ region in which they are permitted for work for say, the first 3 years of their residency?