Wow Jester, you said a mouthful. And I don't think you tripped either. Very interesting insight and most agreeable IMO. I'd like to comment a bit on a couple of things.
You mentioned your close vanilla lady friend who has been naked in the tub with you. I admire your self control and restraint. It that were to happen with me, I'd have a raging erection before you could blink. Then I'd be soooo embarrassed.
I wonder if spontaneity can be planned. If a couple is of the same mind, but wishes not to engage in planned sex with others, rather to enjoy social interaction with others who may or may not become friends and/or playmates, would it be possible and/or probable to have the ground rules and communication guidlines planned should a favourable opportunity arise? Yikes, since it took that long to say it, it might be a lot of work. Might be worth it tho.
My apologies for not addressing the original question. It seems simple to me. Ask carefully. Observe quietly. Listen intently.