I'm surprised your travel agent wasn't able to answer these questions for you.
As for going from the Miami airport to the pier and back, best, easiest, and cheapest way is by taxi. They have a set fare, which I believe is $16 each way if my memory serves me right. May have gone up a buck or two since the last time we were there. And taxis will be in abundance, so there's no need to worry.
As for the 'gratuities', if you don't pre-pay them, then they'll be automatically charged to your onboard account, which in my opinion is the better way to go. If you don't like the service, they'll give you a hard time about reducing the amount, but it can be done with some persistence.