I've traveled extensively with a rear facing big Britax. It won't hit the seat in front of you. Or if it does it won't hit it hard. And Britax allows for bracing anyway.
It doesn't matter if the person in front of you can't recline. When you buy a seat you're not guaranteed a reclining seat. There are tons of seats on the plane that don't recline. The last row, the row in front of the exit row, for example. If the person there is unhappy they can ask to be reseated.
At 18 pounds your seat isn't certified for forward facing use. Not to mention a big Britax is MUCH easier to install rear facing than forward. Forward the buckle ends up directly behind the seat and it's impossible to open. To the point where many of us have either removed the seatbelt from the plane's seat, had the pilot or crew back to help, have had maintenance come and get the seat out, etc. To get it to work you can either flip the buckle, or get a seatbelt extender so that the buckle ends up outside the belt path. Either way it's a hassle. But rear facing you just put it through the belt path and buckle it over the cover and you're done. Easy peasy.
Oh, put the armrest up.
Safe trip!
Wendy