I don't think th RPD was informed about the UBCS, there are no records of them trying to communicate or joining forces. Umbrella's intentionby sending the UBCS into the city was to gain combat data about their BOW's (they placed BOW's around the city on purpose).
As MG said, the UBCS members didn't know what they were getting into, save from the Monitors - wich were all killed by Nicholai (their mission was to recover data, and by killing the other Monitors, Nicholai assured he'd have exclusive information, granting a much higher reward).
About the headshots... you have to understand that, in a panic situation, you'd probably just shoot the zombies anywhere to slow them down, instead of wasting time aiming for the head.