I don't think it was ever specifically stated whether or not the RPD was aware of Umbrella reinforcement but, considering that everyone was in the dark as to Umbrella being behind the outbreak, I would hazard to guess not. Besides, if the RPD had been aware of the UBCS's (Umbrella Bio-hazard Countermeasure Service) appearance, Jill would most likely have known, which she did not.
As for your other question, the Raccoon City Police department had never dealt with anything like this before. Sure some of them would have eventually figured it out but by then it would most likely be too late, they'd either already be partially infected or severely outnumbered - even though the numbers were never really even to begin with. (The police department and a few troops of men against an entire city? Those are hopeful odds; and ammo does not last forever, my friend. Considering the lack of people "in" on Umbrella's foul deeds - lessening the number of potential saviors - and Raccoon's isolated location, the survivors were doomed from the start.)
As for the UBCS, the game was set up so that Jill and Carlos were the only survivors, not to mention that, according to Carlos, most of the people in his unit were simple mercenaries who had not one iota of what was going on; and the line-up of the ill-informed includes Lieutenant Mikhail Victor who was even higher on the totem pole than Carlos. From what I gathered, Nicholai was the only one in Carlos' unit who knew what was going on on a scientific level and, in that respect, the other units were possibly oblivious save for their commanders/aka the other "watchdogs" as well.
They were, essentially, as in the dark as the RPD.