For HDDR it's not the travel time from MK to Ft Wilderness (take the boat, not the bus), it's the time for the show itself that would be the barrier if you only have 1 MK day.
The closest non-park buffet would be Chef Mickey at the Contemporary. DON'T take the monorail if time is an issue - it's less than a 10 minute walk from the front gate of the MK. 1900PF at the Grand Floridian is another dinner buffet option; take the boat to the restaurant, although going back to the park the monorail is probably just as fast as the launch. The bigger time drain at both, though, is if your party needs to wait for all the characters come around.
Would another option for you be to eat your meals CS in the park but do the Wishes Dessert Party? It will NOT give you a good view of the evening parade so if there's only 1 parade the night of your visit and the parade is a deal-breaker, don't do it. But if the parade is not so much of an issue, then the Dessert Party gives you a 'special' experience with the chance to sit down and actually talk to your party, plus a buffet full of yummy desserts to sample. Note that ADRs for the Dessert Party do not open until more-or-less 60 days in advance. If you're worried about it, make an ADR for dinner at someplace like The Plaza but then when the Dessert Party opens, cancel the Plaza if you get the Dessert Party ADR instead. You CANNOT eat a full dinner and then go to the party - make your main meal a good 4 or 5 hours before the start of the Party (starts for all attendees 1 hour before Wishes no matter what time the ADR system 'assigns' you).