1) The food should be double wrapped in aluminum foil for kashrus reasons. Because of Shabbos issues cold food should preferably not be placed directly on the hot plate but rather on an intervening container.
2) A key may only be worn if it serves a practical purpose as part or the garment. So if the key itself, not the attached pin, is holding together two part of the garment it is permitted, otherwise it is not.
3) The vaad that builds the eiruv also takes care of making eruv chatzeiros for the whole neighborhood.