It is your responsibility to call Carnival if you find a lower price. They will send you a form in an email that you fill out stating where you found the lower price and what the price was. When they check it out and find it to be correct, they will adjust your cruise fare if the balance has not been paid in full. If it has been paid in full, they wil give you an onboard credit for the difference amount.
As to your question about priority boarding, both members will get the Platinum priority boarding.