The appropriate time to apply is this summer and fall. I am not sure the exact deadline for this application cycle. My year, the board date was mid-December of my first year and I received the decision in January. Make sure that you are working with an AMEDD recruiter, not a general recruiter. Unfortunately recruiters don't always have accurate information, so you may need to do some digging to get a recruiter who can help you be successful. It may be helpful to contact your vet school to see if they have an idea who students have worked with in the past.
Yes. However, only 4 of those years are active duty. During your 5 years of IRR, you essentially live as a civilian.