Other mistake: In a scene after Caesar picks his name where they are outside then walking down stairs outside the civic center in the plaza (filmed at UC Irvine), you can catch a quick glimpse of students or faculty in the upper right corner with cameras taking pictures.
Other mistake: We know that, after Cornelius and Zira were executed at the end of the previous movie, "Escape from the Planet of the Apes," baby Caesar was only an infant who was just starting to say "Mama." In "Conquest," we know that 20-year-old Caesar was raised (and secretly educated) by Armando, the circus owner. But, after 20 years of being raised and secretly educated by Armando (who has a thick Mexican accent), Caesar has a distinct British accent.
Chosen answer: Caesar wasn't the only surviving ape, just the only surviving talking ape. Conquest of the Planet of the Apes is set roughly 20 years after Escape from the Planet of the Apes, and in that time man began to keep apes as pets (disease wiped out dogs and cats) and (most likely through breeding), the apes evolved into what we see in the film.
Bishop73