As far as guests who observe 1st 33 days attending wedding of someone during last week of Nisson, at time of wedding thou art on turf of Baal[as] Simcho and hence follow his/her Minhog & are allowed to partake in music.
Most people would attend a chasunah, but I know someone whose rov paskened not to attend a chasunah during the time of the sefirah aveilus. I think it was okay to be at the chupah.