But much like with investing in stock I don't know for sure a priori which investment will bring the highest return. If I'm 80% sure that charity A is better than charity B (for my definition of 'better) I'll give 80% of my investment to charity A and 20% to charity B.

When comparing mosquito nets versus theater, I don't really think the stock diversification analogy fits at all.

Some of those things (investments) will give you the dollar back later, and might be considered better ideas on that basis.

But that’s what leads to “earn to give”, which is what leads to deciding to become a billionaire and/or give away other people’s money instead.