It is only when, before the final confrontation with Team Plasma, Colress tells him that Terrakion "is looking for a worthy Trainer" that it all makes sense: just like they put an end to a war between Pokémon and humans in the legends, they sensed the calamities that Team Plasma's exploiting of Pokémon would have brought and actively sought a Trainer to lead them against the new enemy.
Since these games take place two years after Black and White, it makes sense that you would still have access to them. Why the mixed Pokédex? After you beat the Elite Four in Pokémon Black and White, you gain the ability to catch Pokémon from the other regions of previous generations.Why does the Player Character talk when he/she is acting? Because they don't talk anywhere else due to the fact the makers wanted you to imagine their personalities, and in Pokéstar Studios they talk because they're acting.