During the sequence where The Phantom leads Christine down into his own lair, I could swear that the music is evocative of a specific Argento movie.

I don't have a library available now, but is it the opening decapitation scene to PHENOMENON?

Which came first?

Coincidence, I'd say, but PHENOMENA did come first (85; PHANTOM opened in London in 86). I found the PHANTOM movie to be entertaining hokum, just like the show, and otherwise not very Argento-esque (I wonder if Joel Schumacher screened his 98 version of the story?).