Q1 - Well, do it as soon as possible. You really wouldn't want to care about the deadline in this case
Q2 - My wife contacted the doctor who did the initial medical examination for her. The letter from CHC to my wife stated that she had to contact a DMP within 30 days to schedule an appointment. I believe you can do the same, unless instructed otherwise in the letter from CHC?
Q3 - In our case, we were required to have prepaid the courier for the return of our documents. I believe it is the case for all the applicants. At the time of mailing out your passport, you can advise the courier office of this prepaid requirement. They will know what to do.