It's can't be opened directly, Someone with CS6 would have to save it as an IDML. CS5 could open the IDML, but be aware that the text could be re-flowed, and any place that new features from CS6 that were used, the effect will be either flattened or absent.
No. You’ll need to save it back by exporting to IDML and opening that in CS5.
Note that text will likely reflow and any new features will be lost or mangled.