I don't know when Kipleigh Brown and James Kerwin started dating, but if it was before filming on Fairest Of Them All, I would guess one brought the other in and that was when Brown's role was clarified and her involvement in a "sequel to Gary Mitchell" was being developed.
Well, I haven't been on these boards in many years (what'd I miss? lol) but I feel the need to correct this speculation for professional reasons.
Kipleigh and I ended our relationship
many years prior to
STC. We saw no reason not to work together, though, because we always got along well as artists. Kipleigh and Vic both appeared in a short film I directed when
STC was beginning. Vic asked me to come onto
STC for episode 3 ("Fairest"). Kipleigh contacted Vic to let him know she'd like to be a part of it as well, because the first two episodes had turned out nicely. Vic asked my opinion, and I said, "Sure, why not."
My original script for "Fairest" did not name the conn officer in question; it was only after Vic and Kipleigh had come to an agreement for her to appear in that role that I thought "Huh, she looks kinda like Andrea Dromm; that'd be a funny little in-joke if we said this was 'Smith' in the Mirror Universe!" And so we did. It was never intended to go beyond that.
When Chris White turned in the first draft of his script for episode 4, it became apparent that we'd need another conn officer in it as well. UPM (later co-producer) Kasey Shafsky suggested we bring "Smith" back — this time her Prime Universe counterpart — and we saw no reason not to. And that was that.
The WNMHGB connection was thought up by the finale's writer, sci-fi novelist Robert J. Sawyer. It had not been pre-planned.