When your channel launches with deep linking, they would prefer the channel to automatically log in the user (if you have users log in), check if the content is free, and immediately play the content if it is already paid for or free. Otherwise jump into the payment routine. For the CC, if you set subtitles=invalid it should show "No subtitles available" when the user presses the options button during video playback or something similar.
You can set it for the content node for the item to be played at any point before your video playback routine starts up. Usually I would set that immediately before adding the content node into the playback queue. If there is a subtitle file available, populate it, if not, set invalid, and off it goes.