Incidentally, a pawn leaving the assigned area while on the way to somewhere is because only the destination is checked for validity. That is, they will only do a thing in an allowed area, but traveling doesn't matter. I have guinea pigs, for example, assigned to two non-contentious areas. They will freely travel between those two blocks, but will only gaze in an assigned areas. They can't even stop in an area where they are not allowed (under normal circumstances).

Outdoors is outdoors; temperature will be the same regardless of whether there's a roof. Also, cooking outside invites crippling all of your colonists with food poisoning because the outside is inherently dirty. To avoid the problem of the campfire overheating the room, I suppose you could remove the roof of the kitchen. No roof means you can't control the temperature, but as long as you have four walls and a floored tiles, you can at least make the room clean. I've never tested that, however, because I hate playing in biomes without wood or food. And as long as you have wood, you can build passive coolers.