“In any case, I refuse to believe that Rav Moshe would allow mixed seating if he felt it was not appropriate, regardless of who “the other side” was.”
There is a difference between “not appropriate” and assur. You are right that if it assur, he would not have allowed it. But if he felt that it is appropriate to have separate seating but not assur to have mixed seating, it makes sense that he would have allowed it, at least in certain cases.
My impression of the halacha (based on conversations I’ve had with knowledgeable people) is that it’s not assur to have mixed seating at weddings (at least not according to all accepted opinions) but there are very good reasons for not doing so. One reason is, as Joseph mentioned before, that there may be a problem with saying “simcha b’mono”.
The other problem is that I have noticed at mixed seating weddings (or partially mixed seating) that there is very little room for the women to dance. I know this is a practical problem not a halachic one, but it is an important issue, nonetheless.