peanut-allergy mom here...
we fly southwest and are very pleased with their handling of my son's disability (yep, it's covered by the ADA, fyi)... they encourage us to fly the first flight out (great going, not so great coming home LOL) and also allow us to pre-board (no $10 early bird boarding costs for us! LOL) in order to wipe down the entire area (seats, seat backs, window, armrests, tray table, etc) and inspect the area (seatback pocket, floor, crevice in seats where stray peanuts have traveled) and make it safe... they also make an announcement as to why they are not offering peanuts but if any pax must have them, they will be happy to give them to them on the way out!
appreciating their consideration, we also are considerate, when we board we do the wipe down as quickly and efficiently as possible (we've developed quite a routine), we tend to choose seats near the mid to rear of the plane so others can board and not have us blocking the aisle, etc... we also thank the flight attendants for making the announcements and keeping our son safe...
to answer some of the queries... my son is effected by peanut dust, so yes, inhalation will trigger a reaction... a pb&j twenty or even two rows away will likely not kill him (or even trigger a reaction) but please wipe your hands w/a baby wipe or wet-one prior to opening bathroom door, touching common areas, etc (in self-defense, ds always takes a few baby wipes or clorox wipes in his hands to open bathroom door with, and wipe down where he might touch when on a plane... he doesn't do this in every day life like at the movies, etc but he's also not thousands of feet in the air w/no medical help around!)...
a little consideration and tolerance goes a long way!