Hi Arclighted, I'm glad to hear you've enjoyed Sable's Grimoire so much. I'll try to keep this relatively spoiler-free. The new content is coming to consoles at some stage... but it's in the hands of the company who port SG, so I don't know when it'll actually be done.
Rei's new ending isn't a double romance ending. It's not a romantic ending at all (though it's definitely a happier ending for Rei).
Whether other characters (including Rei) get their own games or not is yet to be decided. At the very least it wouldn't be until SG2 is done. Any hypothetical new games for different heroines would take place chronologically after SG2 and continue on from their routes in SG2. So in Rei's case it could potentially be SG1 (rei's route) -> SG2 (rei's route) -> new spin-off game.
That's how I'd see it playing out, though again, it's not decided yet. I won't rule out more heroine-specific games, but with the next two games in the series (SG2 and Project Homonculus) already announced I'd rather hold off on making any promises until they're done.