OK, I am definitely not a posuk, in fact, I’m a woman who isn’t even a Rebbetzin, but I’ll take a shot at your questions.
1. Why ISN’T it permissible to attend the simcha? Just avoid the sanctuary, which is where we’re forbidden to go, not necessarily the simcha room. I know of many Orthodox couples who in years past got married in conservative shuls. It was commonplace up until the ’90’s in some communities.
2. What do the cousins do during the dancing? Yes, some turn their backs, but to be honest, I’ve never paid it much attention, and we’re generally seated pretty far away from the dance floor. Of course, it’s one of the rare opportunities that the frum cousins get to have mixed seating LOL. But none of us join in the “obligatory Hava Nagilla” set.