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You can gloss it with 'under observation' I think fairly well. But the problem with

(1) She would have had to have been observed.

is that it misses out the gerund form preceding 'observed', which alters the meaning slightly in my opinion. It's difficult to get at precisely but I feel like the original sentence has a feeling of time-boundedness around when she was observed which you don't get without that gerund. I think?

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(native speaker) I agree.

The original with "been being watched" feels like something the detective says when he first discovers the fact about watching. It has an exclamative aspect to it. There's a present sense of realization, mixed with the past sense of when the watching had taken place.

"She would have had to have been observed." is something like a lawyers distillation of what happened later for the court.

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