"...would you say FUCK HIM/YOU AND THE HORSE HE/YOU RODE IN ON! to Frankie Gavin's face?"

I would, given the opportunity.

"I wonder how you would have reacted to hearing that the Canadian band The Guess Who played at the White House for Richard Nixon"

Probably similarly.

"re De Dannan...are they still together?"

Nope. Their last recording was "Welcome to the Hotel Connemara" (2000).