Ironically, solving malnutrition and malaria will lead to an increase in population, which makes solving the problem of Global Warming even harder (and if you don't think overpopulation is one of the key issues when it comes to climate change, then I'm not sure you're able to have a rational discourse on the matter)
In interconnected systems, there's an opportunity cost to opportunity cost.
Depends on the timeframe you're looking at. Within a single generation you're correct.
Once you step into more generations though, fertility rates drop as child survivability rises, as it becomes much more viable to rely on a few children as insurance for old age. Source: Hans Rosling, Gapminder
You're not wrong, but I hope we can all agree that "let millions of people die to slightly reduce global warming" is very far down on our list of things to try.
In interconnected systems, there's an opportunity cost to opportunity cost.