Robin has a point on the weight situation:
The FAA CAMI study showed conclusively that once a child reaches 40 lbs, the lap belt protects him just as much as it does an adult. Where carseat safety is concerned, height does not matter on an aircraft, whereas it does in a ground vehicle. For safety purposes weight is the only thing that counts on an aircraft in flight.
The right to install a seat rear-facing according to mfr. recommendations is written into the Federal law regulating carseats on aircraft. I recommend printing out a current copy of the law and putting it in envelope taped to the back of the carseat. If the FA objects to the placement, you can (very politely, of course) ask her to read the law and consult her FARS handbook on the subject. The law is in the US Code of Federal Regulations, at 14 CFR 121.311
Most kids can comfortably sit in a FF carseat in coach until they are about 44 inches tall. After that there is usually not enough leg room (depends on whether they are long-waisted or not, and how much the person in front of them reclines.) Most of the time their legs are a bit too short in the thigh to be able to drop straight down at the knee over the edge of the seat, and will tend to stick out at a bit of an angle. If the space is tight and the seat in front reclines, their legs may be caught between. This happened to DS once when he was about 3 on a flight to London; in the middle of the night the man in front of him suddenly reclined all the way back. DS legs were painfully pinned -- the resultant frantic screaming woke the entire plane.
As to seat kicking, I have a remedy for that that you can try.
"Hobble-pants" are my name for a kick-restraint trick I came up with that I only had to use a couple of times -- after that the mere threat was enough. (I used them in cars, too.)
What are they? They are a REALLY too-big pr. of drawstring pajama pants, too long by about 18 inches. I put them on the child and then tuck the ends of the pant legs under the tush when I strap them in the carseat. Just like with putting hobbles on a horse, kicking forward beyond a certain point is impossible, but the feet move freely from side to side. Restrictive, but not really uncomfortable as long as they are long enough, and safe for an emergency because they are not fastened to the seat in any way.
You can do the same thing with a blanket tucked around and under, but it's more complicated to tuck it just so and get it the right length, and it is easier for the child to remove, too.