could I change my main doctor if I didn't like the one I got assigned to?
You will need to talk to your health care team leader or to the patient advocate at your VA medical center to take care of that
is there a backup plan for  when someone from the pact team is missing?
Your health care team will have a plan in place if your primary care provider is away or leaves VA. Your team will make sure you always have access to a qualified primary care provider and that you keep getting the ongoing care you need to continue with your health care plan.