looks like more of a memorial service than a true funeral to me. you know, like if they didn't have a body to bury. consider with that that there's no disturbed ground surrounding the headstone...
kitty's age doesn't really matter, considering it's a new actress and kitty played no real role in the previous movies. beast needs only, say, three minutes to change from human to beast. you know, since it only took him drinking a special serum (which he may or may not have developed before/after his appearance in the previous movie.) i don't think there's any justification for saying any long amount of time has passed, or any reason to believe it's NOT jean's funeral/memorial service. one of the founding members of the x-men "dies" (for all they know,) but they just pass it by as if nothing happened, only to have a faux-funeral for someone else later?