No, a subject is either a vassal, a march or a tributary.
It should be possible.
Yes, as long as they're Protestant or Reformed.
they are (the only?) exception. Subjects can't form other nations usually, as the requirement is usually 'independent'. Prussia subject can form from anything, as long as they have right religion and all other needed stuff.
Dunno if there is other exception.
Thanks. I'll continue on my european domination then. (Austria, Bohemia, Poland and Sweden as PU with Russia with a de Valois heir)