I can't speak to Question #1 but my thoughts on Question #2 are that (a) you only get the miles per cabin, not per person. If you read the Additional T&C carefully, you will see:
"Additional Terms & Conditions
Award miles are per stateroom and are awarded to only one passenger per cruise booking. "
(b) I value miles at about $0.01/mile but I know many others value them at a higher rate. I would rather go to a website like Cruisecompete.com and find a travel agent who will knock 10% off the cost of the booking (so a saving of $70 in your case) vs. $60 worth of miles.
Just my opinion though.