I have also found helpful strangers at the airport. This man, who was traveling for business, totally helped me get my stuff through security once when DS was flipping out on me. I've also had people offer to help me with the car seat on the plane. I definitely would take the seat on the plane if you can swing it for the 2.5 year old. He is strapped in and secure, that is one less thing to worry about for 3.5 hours.
I'd get my stuff to the row (people who look like they might be parents or grandparents are usually pretty helpful, if you appear to be struggling, let them help!). put the armrests up. Install the seat yourself. recline it a little just to install it easier. I never have had to ask for an extension (make sure to twist the lever-side 1/2 turn, so that it faces down, otherwise it's impossible to remove your carseat).
L, mommy to my one and only, super-sweet boy, G 6/08
I'm pro-big bird, and I vote.