How are you so sure that disease vector mosquitos don't play an important role in the ecosystem? I would posit that predicting ecosystem outcomes is no easier than predicting weather.
They actually do. They protect the environment by culling local pests like humans.
do you mean the niche of malaria transmitter? I suppose it wouldn't be that hard for another species of mosquito to become a transmitter.
Anyway, out of curiosity i once spent some time correlating genetically modified mosquito releases with the modern Zika outbreaks (there weren't Zika outbreaks before 2007 according to WHO, only like 14 separate cases) - the Zika outbreaks started in 2007 and for the most of them it was possible to find information that there were a release of those mosquito in the area of the outbreak 1-3 years before the outbreak (in the case of Gabon 2007 outbreak it was those mosquito development and testing in previous years on an island off the cost of Gabon. Another 2007 outbreak - Yap island - also seems being a testing ground in the years leading to the outbreak).
This has been studied, though. No animal only eats mosquitoes, and I’d only start with a few species (but likely expand to all of them.)
This is an example of score insensitivity. How many humans die each year of malaria? If this was in Europe or America we would already have done it.
DDT was supposed to be a risk-free way of killing mosquitoes. Nobody knew the side effect existed.