With regard to Stewarts,
The Scottish royal house you refer to are the Stuarts.
As to Russian and Byzantine lines to the House of Windsor, Queen Victoria and Kaiser Wilhelm were known in their day as the Grandmother and Grandfather of Europe, by virtue of the many children they produced (not together, of course!) being married off to practically every royal house in existence at the time. They were, themselves, carrying on a long historic tradition. Look at any European/Scandinavian/Russian/Slavic blueblood's ancestry, and it's almost incestuous, because of the imperative that noble blood does not marry outside its parameters. At last, these days, there are far more instances of royals marrying commoners, with the imprimatur of their families, notably Crown Prince Frederick and Mary Donaldson, and HRH William and Catherine Middleton.
The post-independence Greek royal lineage started off as Bavarian, then Danish. These strands, in essence, explain how Prince Philip got his nickname of Phil the Greek, which he hates.