Some PB students at UCI have had the ability to directly matriculate following the PB. These occur under a very specific set of stipulations and ultimately it is Admissions choice on whether or not they deem a PB suitable to transition directly into medical school. However, I want to emphasize this is not a guarantee by any means merely a possible opportunity if it is presented.
Our program director was very mindful of how the CARS Kaplan course would affect us in the Fall. So even though at times it was stressful, I found it very manageable. Especially since I did research which was 4 units and took only two additional classes to bring my total unit count to 12.
As far as when to take the MCAT, you are right on the money. You SHOULD NOT take the MCAT if you are ill prepared. If you do not get a competitive score that could realllllly hurt you in the long run. Our program director who works in admissions advised us that different committees at different schools asses MCAT scores differently. Some will take a composite score ,or look at the most recent, or look at your best score more. So if you are not prepared and you dont excel this could really hurt your chances in the long run. However, this is not to say a "bad" score eliminates your chances to get into medical school. It just can create another obstacle for yourself. I remember when asked in my interview if I was taking it, I told her I was considering on studying and taking a practice one before the PB. She said NOOOOO and advised me to wait and do the program if I were accepted.
Your head is in the right place. In my personal opinion, it is far more realistic to go into the program with the intention of becoming a stronger more competitive medical school applicant. The relationships and connections you build are invaluable. At the end of the day, if you work hard and excel it will pay off in the end and many "PB friendly" medical schools will take your PB year strongly into consideration.