3) True. You have to get the gate number from your handling agent and tell the ground control what it is when you check in with them when clear of the runway.
4) No you cannot choose to opt for voice over CPDLC once you have logged on but I guess you could just not log on. However, I cannot imagine why you would you want to do that. Life is so much easier without that RT hassle, particulary HF, and for all clearances you have a hard copy so less chance of getting something wrong.