A. Yes. Michelle Trachtenburg, who plays Dawn Summers, was born on October 11, 1985, so her 16th birthday was October 11, 2001. The musical episode of Buffy - properly called Once More, with Feeling - was first aired on November 6, 2001, meaning she was certainly over 16 when it was broadcast. (And no, I don't know when it was filmed exactly, but I do know that Buffy episodes tended to be pretty close to the air date.)
However, if you live in a country in which the legal age of consent is over 16 then you're a paedophile if you perv over Dawn in that particular episode.