] I would ask here, What about Horatio?

I complete agree, as I said just above. I should learn to read posts in the right order! 8-/

] In fear an trepidation for daring to argue with Derek Jacobi - what do I know?? :-}

Oh, pish tosh, honey! You've studied this play enough for your opinions to matter just as much as Mr. Jacobi's. It's just another idea among thousands...

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A soliloquy is certainly meant to be "talking to oneself" and to give the illusion of speaking aloud "unspoken" reflections. But I also maintain* that if a character is not alone on the stage, an otherwise straightforward "soliloquy" changes quite a bit. There are no stage directions in any of various texts of Hamlet that say that Polonius, Claudius, or Ophelia actually leave the stage after the "walk here, read this" speech. That does not prove anything, of course, but it's something to keep in mind when trying to work out how the beginning of the third act is meant to come off.