Expat...

An expat, from several authoritative sources, is someone living outside their country of origin. To quote the American Heritage Dictionary of the English language as an acceptable source -

Expat
1. An expatriate.
2. An expatriate; a person who lives outside his or her own country.
3. A person who is voluntarily absent from home or country.

Citizenship is not usually a factor and expats are often temporarily in a country for a year or more for work reasons. They do need to apply for fiscal residency permits, which in Spain are issued for 5 years at a time. When those expire one can apply for another, or apply for citizenship, whichever.

Citizenship involves taking an exam in the language of the country testing knowledge of both the laws and history of the country. So I'll be applying to extend my residency, then ... I do pay full taxes, in fact as a fiscal resident I can even vote in local elections, but not in national ones.

As for Canadians moving down for their permitted 182 days before they have to pay taxes - in the same blog you say your son-in-law became a citizen in "less than a decade". That's the thing. American citizenship is neither quick nor easy. It also carries a tax requirement for life. Actually any American seeking to become an expat should also be aware that the US government will tax them, too, for the rest of their lives, even if they move and take citizenship in another country. Not the only government to do so, a few non-EU others do, but quite a few American expats here hadn't realized and although their Spanish taxes are taken into the reckoning, they are still forced to do dual tax returns every year even though every penny is earned in their adopted country. So those Canadians have some factors to weigh up while they are there spending money in your community, paying rates and local taxes, and generally paying their share into local prosperity ...

Since your blog was about filling in gaps, hope this helps with that.