Also, GiveWell came to the same conclusion (excellent charity, but can't currently utilize marginal funds) last year. Also, ancientcampus (and anyone else who cares about effective charity) should probably wait until they refresh their rankings on December 1; this is the time of year that their current views and wobsite recommendations are furthest apart.

To answer the OP's question 2 in more depth, any of the charities recommended there would be excellent candidates: GiveDirectly and the two deworming charities (which you pick will depend on which is tax-deductible in your country). GiveDirectly is certain transparent and efficient, in that it simply gives a large unconditional cash transfer to poor families, with around 90% 'efficiency' in the sense that 90% of your money ends up in their pocket. See this accessible introduction and this detailed analysis.