I think you'd be very, very lucky to make it to Fantasmic, at all, with that schedule. First, it would be unusual to get seated right on time at those two buffets -- very popular places. Even if you're in & out of Crystal Palace by 6:30, you then have a long walk to the busses, followed by the ride to DHS, then going through bag check/turnstyles...then a long walk to the F! theater. I really doubt you'd get to the theater by 7:00 p.m...I would guess it would be after 7. Chef Mickey's usually backs up even more than CP, so don't count on getting seated at your ADR time, even if you check in early. I've read about 30 minute waits at CM being common.
And sorry to say, even during slow times of the year getting to F! less than an hour before show time could be too late to get a seat. We've been there during slow times when 45 minutes before showtime was standing room only -that was for an 8:00 show, too. Problem is, during slow times, they cut back even further on the number of shows - sometimes only once a week. So while the crowds may be low in the parks, EVERYONE in the parks will go to DHS that night to see Fantasmic, because it's the only chance they'll have.
I'd use the table service for lunch that day, and do a counter service meal and get to Fantasmic theater 75 to 90 minutes before show time, just to make sure you have a seat.