mpapdp.1984:
1. If you feel that PCC's will take some time to process, you can send the rest of the documents and put in writing that you will submit the PCC's as soon as you get them. Other way is to do is to wait and send all the documents together. Both work fine (there is no right or wrong way here).
2. You don't need to jump the gun as far as the interview goes. It is very likely that your interview will be waived. Hypothetically, even if there is a 1 percent chance that you will be asked to appear for an interview, you can request the CIC Buffalo for postponing the date. However, their decision will be entirely based on how strongly you present your case to CIC (for example, letter from the doctor attesting to the fact that your wife can't travel under adverse conditions and/or hospital records showing her final stages of pregnancy and her inability to move under duress). I know of at least 2 applicants who were granted reprieve based on the strength of their case (family emergency).
It is a good sign that your case is showing some "movement". When the e-cas shows "In Process", it usually means they have found your application eligible to be processed for immigration. The last hurdle that you need to cross now is medical examination. Hopefully, you will get the medical request soon. Hope this information helps. Good luck and cheers.
Mark