9 Rhodium

RE: Dell Inspiron 5521 | RAM Upgrade | Memory Module

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).