Very well written post! Major kudos on not wanting to drag a new person in to a messed up situation.
If I could offer one piece of advice, assuming that you and RS do decide to open your marriage, it would be to go to munches, play parties at your local dungeon, and local conferences, campouts, and other kink-lifestyle events. I live in a major metropolitan area, so I may be spoiled by having all of these things available. If you're not sure what the opportunities are in your area, you can always set up a fetlife profile and explore.
It's great that you have a past experience under your belt and lots of hot ideas, but I think that any relatively rusty/new kinky person would benefit greatly from meeting and talking to like-minded people, watching them play, and getting a feel for what possibilities are realistic, safe, sane and *awesome*.
These experiences will not only help you refine your ideas and techniques as a dom in preparation for finding that special sub (nothing is hotter than competence!), they will also expose you to lots of friends and like-minded folks who can counsel you through your poly problems. You may even find the lady you're looking for there, though I recommend going with learning in mind rather than dating, at least at first.
I think that saying "I will not a date a woman if she's not someone you can get along with and potentially become good friends with" is a fine criteria. You can keep things very casual with new women until they've "met the missus." But you can't *require* RS to be friends with anyone new. Friendship is tricky, just like love is, and it has to grow organically. I also think having her pick someone out for you doesn't make a lot of sense... after all, chemistry and compatability between you and your new partner has to be high for something to blossom, and who can really feel and identify that aside from you?
Good luck to you both. You shouldn't have to remain indefinitely unhappy.