The old booking engine showing no availability on CMB-ZRH is something I encountered as well a few weeks ago in a multi city booking. My solution: from Swiss's mobile app, look for a return trip ZRH-CMB-ZRH on the dates you want. Add them to the cart and go all the way to the payment step (incl. adding your passenger information).

Once you are there, the system creates a temporary PNR. If you exit the booking procedure, you'll see in your trips' list that the new flights are there. You can't check the details of the flights because the booking hasn't completed, but that's not an issue.

Now, call Swiss with this PNR, ask them to add the 2 segments LHR-ZRH and ZRH-LHR when you need them, make them price it, pay, and voila.