Whether Sidwell stays or goes shouldn't affect player of the season. Sidwell for me - Stockdale hasn't played enough to be in the running, in my opinion. He's certainly one of the few to come out of this season with much credit.
There's a certain element of romanticism in my reasoning Captain; goodness knows there can be little footballing reasoning for choosing a highlighted player of such a bad season. If Sidwell decides to leave us at the end of the season it would leave a bad taste (although on the flipside if he gets an offer from a PL team it would objectively difficult to begrudge him that). Stocko has been the fans' representative on the pitch and shown as much, or more, passion and commitment to the cause as anyone in the short time he's been in the side. Diatribe aside, you're right, Sidwell should get it for the reasons you underlined.
And the DCM (Don't Come Monday) Awards for 2013 should go to Taarabt, Berbatov and Ruiz in recognition of leaving in January. Thanks guys!
To be fair, leaving (thus freeing up a loan slot for Holtby) was the single best thing Taarabt did for us.