[bonchicbongenre]

I very much agree with English being a hodgepodge of confusing rules, especially those tense-related.

Re rewtriting (3) though: does not the following suffice? "She must have been being watched".

I can imagine an argument that "She would have had to have" is not equivalent to "She must have" on grounds that the "have had" might suggest additionally that the past-ongoing-watching no longer occurs. I don't really think this flies, though, since the natural reading of "been being watched" already suggests to me that the watching was relegated to the past ("been" being interpreted as only connoting the past, not the "inclusive-or"-type interpretation "previously and perhaps presently"; much as "or" itself is - to my chagrin! - generally interpreted exclusively in standard parlance).

I couldn't think of how to simplify the "been being" though; that is a tough nut.

[penteract]

The "would" makes the claim conditional, so you can't get rid of it without potentially changing the meaning.

[boloust]

Someone must have been watching her