Personally for a 7:30 ADR at Chef Mickey's I would leave about 6:15 that way even if there is traffic you have pleant of time plus if you get there early you could always check in a little early as they might slip you in or you could always go shopping or look around.
As for getting there, I would go from your resort to MK then either walk, monorail or boat over to Contemporary. As for goind back, that all depends on what time you get done at Chef Mickey's and what time the MK closes. If you get done and the park is still open just go back the way you came.
Hope this helps.