I'm pretty sure that when a child is under two and a seat has been purchased for them on an airplane they are required ot have a car seat. (Jel would know this one for sure I think).
As far as Included v. additional charge...
To and from the airport is not included. You have to arrange this through Mears.
Once you get on property, all transportation on the boats, busses and monorails is included. of course if you chose to take a cab it would not be.
Also, I'm not sure whree you got the idea that the transfer was 20 minutes, but that's not really accurate. If you are driving or taking a car service, it's still a full half hour from MCO to WDW, depending on which resort you are going to it can be even longer. With mears you'll be put on shuttle bus with people going to a variety of hotels, and very likely your "tour" will be over 45 minutes, and as long as two hours!
You also mention that you have a lot to pack--maybe there's a solution for you. I assume a lot of what you are packing is diapers and such. For about $30 more you could take a car service from MCO to WDW. (I like yourride.net) They will stop at a grocery store and you can buy diapers, snacks, juice boxes, things like that. It will be a lot more relaxing, only you and your family will be in the car. You can tell them you need a booster for your older DD, and they'll supply it for free.
Something to think about.
Anne