I can't speak too much on the specific questions, but I can say that I'm sure the F/As will be happy with a bag of chocolates to share among the crew. I've done it within the past few years for the gate agents when I flew rev on BA. Turns out they already had a new seat assignment for me in E+, since my seat had a place for a bassinet which was needed for someone else. Just make sure that it's sealed, whatever you get. Ultimately, the best you can really do is ask nicely and hope for the best. Maybe a quick badge flash or something, but don't try to make it seem like you're expecting anything. Ultimately, a gate/check in agent will do what they can to get a family sitting together.