There's no way the release schedule would support more than one per season.
Wondered whether they could stretch to two per season, but completely forgot that season 6 would have three two-parters, which would seem highly unlikely.
Ah well. I imagine we'll be getting "Unification" as that would seem to yield better extras opportunities than "Time's Arrow", and then "Chain of Command" and "All Good Things".
I really do not think so - the reason is simply the standalone releases are there for buyers to double dip. They are using marketing opportunity and preparing a release for people who do not want to wait another few months for 2nd part of the 2-parter.
Releasing 2 episodes, which are already both on the season set does not make so much sense, despite there still would be a small audience of random buyers or casual fans not wanting full season set.