What I did was:
Started the same way as you, but subduing and hiding the chef, but lured the guard on the bench upstairs straight away. While he was going up I needled the guy who needed to be poisoned and legshot-subdued the other guy. Then I grew the mixtape across to caruso's room (this was the hard throw: I needed to lure the shotgun guy and not his friend) and subdued the guard (who should have been just at the cliff by now) with a thrown crowbar over the railing. I then ran to the girl and subdued her (she was coming up the observatory stairs, investigating the noise made when I legshotted the housekeeper) and went outside to push the bosygiard off the cliff. Then I went back to the observatory roof and waited for the guard to go back to the staircase that connects the first and second floors, subdued him there and dumped him off the roof. Ran back to the girl, shotgunned her, and hid her in a box in the observatiry. Didn't get past that point as I was spotted on the way down.