Ive seen people but boosters in the overhead bin, so id think they would let you put a radian.. especially since if they even realize its a car seat, they probably will think you are going to install it. When you whip up into the bin, and you can barely tell its a car seat, i doubt much will be said. Especially if its in the back bc they always announce "Overhead bin room in the back is full, please use any overhead bin you pass that has room"
I think you will be fine finding the 4 seats, even if you board after the A group. When they redid their tickets and such and made the "business class", it took away the first.. 15-30 A boarding numbers (you line up via A, B, or C and by the number (1-60) now..) Generally the business class isnt sold out, so even tho you have A45 there were only 5 people in A 1-30.. I doubt too many people will pay extra for the early boarding.
Im not sure how I feel about the kids and parents boarding after A.. I dont travel with children often... but I fly in and out of Orlando ALL the time, and sometimes it got to me when I was early in A group and had 60 people board before me bc 6 adults/teenagers boarded with a 4 yr old. Obviously someone with a couple of kids, especially alone I have no issues with. I just think a lot of people abused the option. I rarely see people board after the A group, but in the kids group that have trouble finding seats together..
Enjoy your trip!!