The only conclusion we can come to is that Jenny didn't see the priest. Evidently because, as you point out, he was hidden behind the door. This, plus her state-of-shock prevented her from seeing him.

Jenny had to sleep in Vanessa's bed once Bernard moved in. Only Vanessa is shown naked on one occasion; on another she has a short nightgown on. Being sisters, I guess Jenny's nudity was irrelevant on that particular occasion.

It was a song from the youthful days of Meldrum (the psycho priest) and Miss Brabazon, back when they were in love before his mother coerced him into the Catholic priesthood.

Being a Catholic minister, Meldrum had to live a celibate lifestyle, but Jenny stirred up his sexuality since, yes, she reminded him of Miss Brabazon three decades earlier. Evidently, the blonde from the opening, Valerie, did the same thing.

That ties into the message of the film: Authority figures, and especially respected ones, can more easily get away with audacious crimes simply because of their honored position wherein people naturally assume the best of 'em. It's similar to Nurse Ratched in "One Flew Over the Cuckoo's Nest."

It was shown earlier that the therapist gave her medication, a sedative.