Is that descending figure that opens the overture to Ruddigore (1887) a deliberate callback to Saint-Saëns' Danse Macabre (1874)

Hmm, could be. It's not long enough for me to have a definite opinion. But SUllivan definitely did things like that. I've always assumed the very beginning of the overture was a reference to the overture of Il Trovatore . . .