Yes that's correct. You'll get normal miles via the CC and your MPC account will show "Bonus" miles being posted as well, this is the extra double miles.

I think as long as the cardholder is travelling on one of the tickets, you can benefit for all the tickets in the transaction. Often if I need to buy multiple tickets, I will just buy them in one transaction online, get the double miles for all, and then call CX to split the PNR later so that the bookings become separate. That works fine too.