On trills and turns, in order to change the pitch of the note above or below, in printed music, accidentals are displayed right above or below the trill or turn. I don't see how this is expressed in MusicXML - the trill-step or or two-note-turn attributes don't seem to serve the purpose, and accidental-mark is either a notation or ornament in its own right instead of being attached to the trill-mark or turn (or delayed-turn, etc.) ornament.
<accidental-mark> element refers to the ornament that precedes it when used as a child of the
<ornaments> element. See for instance the accidental-mark example or the ornaments description. The attributes that you mention represent playback, and are not widely supported in current applications.