As soon as you get to the gate (early!!) let the attendant know that you need to install a car seat and ask if you may pre-board with the priority group so that you do not block the aisle and slow down boarding. It is tight quarters as it is, but add the car seat and 100 people and I get very flustered very quickly! I was only denied once, on AA (they stink at accommodating families) and lo and behold I had a FA trying to hurry me to get the seat in so people could get by. I made a point to “apologize” later when drinks came and told her that I did ask the gate attendant if I could pre-board so I wasn’t holding up boarding and I was denied.
You mentioned a harness in another post, if your kiddo is old enough for a harness they don’t need to be rear facing. It really is more of a hassle for you because you can’t reach around it or under it, can’t use the tray or in-seat screen, etc. Because in a plane you are more worried about up and down turbulance, not a head on car crash, rear facing really isn’t any safer once they have strong head and neck control and can protect their airway.
My advice is to keep babe buckled in the whole time. Once you let them out you’ve opened a can of worms. My DD will sit happily for a good 4 hours, past that we try to do a lay over, but even just a getting out for a diaper and right back in is a good little stretch. We treat it just like the car, you stay buckled in the car all the time, same with the airplane (and same goes for our big kids too, just good safety habits when flying)