Basically the cruise lines would be happiest if there were NO single cruisers, after all they get less total revenue per number of square meters occupied etc. I suspect they do offer single cabins etc because it would be bad publicity if they did not, no other reason. In your example the best bet would be to go cruise only and make your own flight arrangements.
Why not try cruise lines whose ships have a reasonable number of single cabins? They are more expensive than the individual cost of double cabins. There lines include Fred. Olsen, SAGA, P&O and, I belive, Cunard.
The answers simple book the cruise only & arrange your own flight.
If its cheaper to pay for 2 to go im free what date are we sailing oh and thanks .