-The start date is the Monday of the first pay period in October, in this case October 3rd 2016. This date is not flexible baring some sort of extreme extenuating circumstance.
-Similarly the end date is the end of the first pay period in October. (This hasnt been set for 2018 yet) This date is much more flexible, generally you're considered to have completed the program in May of your second year. At that point most associates look for other jobs or look to school. If you're hired by the CDC they will make an offer by July and you will convert to the new position after the official end date in October. If you were to resign in Aug of your second year to attend Medical School, then no one would fault you. People leave the program all the time, a 2014 assocaite at my host site left last month for a similar and better paying position at a health department in her hometown. Two people from my team (2015 cohort) have already left for other jobs, one in the private and one in the public sector, and no one was upset and they can still put CDC on their resume.
-Finally, you will most likely not be replacing anyone. Your host site might not have had a PHAP before you, they may have other PHAPs that you will work concurrently with. or you may replace a PHAP but work in a completely different area. As such they don't put you in contact with anyone, that said when they tell you where you are feel free to look up on the PHAP website if there are other PHAPs at your host site and reach out to them on your own.