Alternatively, can we nudge evolution in a desirable direction so that the disease spreading capabilities of modern mosquito's are surpassed by a new species that serves the same ecological role without spreading disease?
Impossible. Even if we could nobody knows the full extent of the ecological role of mosquitos. When you are talking about ecology there are too many variables to know them all.
Nature lost species all the time throughout history, and the world survived. I'd say, there are so many variables that the ecosystem can surely adapt to the loss of any particular species.
