I think the ES rate is only available up to 3 months out for shorter cruises and up to 5 months out (I am pretty sure) for longer ones... so after that time there will no longer be an ES rtae, only the regular fun saver type thing.
As far as booking through an agent vs. carnival directly- if you go direct w/ Carnival you will have to keep checking to see if the rate drops- if it does you fill out a form from the carnival website and get either $$ off the amount you owe (if before final payment) or as an OBC if after final payment date. A good travel agent will keep checking rates for you- and the TA would need to submit the form, not you.
You have to be very careful because some agents will only allow 1 price drop- after that they charge fees that may eat up any price drop savings. Carnival will allow you as many price drops as you are eligible for- right up until 2 days before sail date.