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I remember seeing an article in the Waterlines publication extolling the use of loose polystyrene beads to form a barrier layer that floats on the surface. These may be hard to obtain though and not a 'quick' solution.
The most common method to prevent mosquito breeding is to prevent access to the water using netting over vent pipes and making sure that lids and access covers are sealed. Oil barrier layers are very effective but not so healthy and must affect the taste of the water - this may be a question of 'if it is acceptable and works then leave it alone'. Probably best to ask who introduced it and ask them why this was chosen as the best way to tackle the issue.
A most effective way of reducing the incidence of malaria is through residual permethryn (treated nets, cloths, curtains, porous surfaces) in the home and on clothing. Have a scoot around on the 'net for a publication on using treated Mbu Cloths in Kenya - very effective at reducing biting and incidence of malaria in villages. An important aspect is how many people adopt measures - as in so many sanitation issues - you can protect yourself and your family but your neighbour's behaviour affects your health as well. But the mozzies in Ivory Coast are apparently resistant to all available insecticides (see the liv.ac.uk poster link below - so physical means may be the only method available to you.
The batteries are a real issue - heavy metals are cumulative (rings bells but not sure if cumulative in the soil or in organisms such as ourselves) and will contaminate the soil and the water table. If rats are eating the black powder then it is getting into the food chain that way too. Small children ferreting about in the dirt will be particularly at risk - I do not know what the immediate effects of ingestion would be though.
Sounds like you are tackling difficult-to-change behaviour issues. Influencing through education, peer pressure and through systems backed up with sanctions may be the answer. Try convincing the influential people first and ask what they would propose as measures to control the disposal of dead battery cells and reducing the mosquito population.
Even in the UK it is hard to convince people to not throw away dead battery cells into the general waste stream. The EU is imposing requirements on member states including limits on percentage of waste comprised of batteries and accumulators. Maybe a 'take-back' scheme linked to something attractive could be started? Depends if there is a market for dead batteries and on Government interest in preventing pollution and raising the standard of public health.
I google'd 'vector control ivory coast' and found a bunch of potentially useful links including https://www.lshtm.ac.uk/aboutus/people/Rowland.Mark and https://www.liv.ac.uk/media/livacuk/pgr-development/posterday/winners2013/Edi_Poster,Day,online,(1).pdf
What a tricky pair of problems you've got!