1. Yes, albeit at a performance penalty. It's designed for a pair of matched modules.
2. Either will work by itself. Don't mix DDR3 and DDR3L though -- if you have DDR3, use that; if L, use that if you're adding a module.
3. Crucial is hard to beat particularly given its compatibility guarantee when purchased through its memory configurator. Anything else- ask the vendor about compatibility guarantees (or be sure you understand the possible restocking charge if a return is required).