1. Interviews are scheduled based on when CN become current.
2. IF there are health concerns likely to prevent the applicant from getting a visa, the Panel Doctor will inform the applicant at the time of the interview, the doctor will also recommend an appropriate course of treatment. They will not go ahead and simply issue a medical report if the required course of treatment is not completed.
3. Regardless of the POE indicated at the time of your interview, you can enter from any other point whenever you decide to make the move. Normally, I assume you wouldn't open a letter that is not addressed to you, or that is addressed to your spouse all under the guise of openess, democracy and freedom, right? Well, same with the immigration package that is addressed to the US Immigration Officer. They insist on it not being open to guard against people tampering with the information contained in the packet, it could also be to ensure nothing goes missing from the packet. There's no telling what fraudulent practices some people could resort to if they're alllowed to open the packet.
You can read more about the sealed packet from 9FAM 42.73 (pg. 8) below
http://www.state.gov/documents/organization/87909.pdf