Comerei naquela hora should work, didn't it?
"Comer em" usually implies space (Vou comer em casa), "Comer a (determinada hora)" implies time (Comerei ao meio-dia naquele restaurante).
is the difference between aquela and àquela like adding a to aquela [ a aquela]? so we get at that hour?
is there some slight difference in pronunciation?
Yes, "àquela " means you have a preposition before. The pronunciation is the same.