CS5 is not Retina compatible. You can upgrade to CS6, which is.
Standard for $200/Extended for $400. Be sure to edit the shopping cart to get the Mac edition. Windows is the default.
Oh no. That sucks hard. The invisible hand hustled me once again.
If $200 is too much, you can rent the latest Photoshop for $10 a month, and get Lightroom 5.7 in the deal.