[Zombie Cinema] Some rules questions

Choosing sides in conflict: I play it as free and clear, yes. First one player challenges and another accepts. Then there is an expectant pause as the other players weight the odds and decide whether to participate; often this is broken by one or both of the committed players, as they advertise their intentions and how cool it'll be if their character wins. Then the other players choose sides as they will. I've never had it happen, but if a player declared an unwillingness to choose sides before others do so, then I'd use the order of action - in general, I only use the default order of action when it is needed, as it slows down natural communication unnecessarily otherwise. Mostly it comes up when several players want to save a character by sacrifice and we need to find out who, if any, actually commits to it.

Supporting allies: yes, you can loan dice to any player in the conflict if you want. This is mostly useful if an ally in the conflict has better moves than the main participant; you want to increase the chances of the ally getting the narration rights.

Tie on the winning side: here I usually play it by having the players co-narrate, but if they disagree, the active player makes the call. In case the active player is on the losing side, he's stuck narrating his own loss, which is not the end of the world. You might consider this as a stick to keep the tied winners cooperating: they lose their privilege if they fail to cooperate. Hasn't really happened to me ever, though.

Sacrifice narration: Sacrifice entitles you to the narration rights (and a scene if one is not on-going). However, this is a privilege, not a duty; you don't have to narrate events that show your character being the hero and saving the other guy if you don't want to, or if the story can't support it right now. You may also choose to make the narration metaphorical, like making it a flashback to how your character supported the other guy's confidence in the past or whatever. Basically, this narration only exists to prop up the protagonism of the sacrificing character if the player wants it, but using that opportunity is not mandatory.

Advanced sacrifice: Everything you describe is rules-legal. In fact, if you ever play with more than six players, I definitely recommend a "GMing gambit": take the first turn and describe how your character almost prevents the zombies from escaping, then sacrifice-kill your character and give everybody else a leg up. This way you can reduce the relatively high number of characters without much fuzz, and get to act as a general GM player instead of having to worry about your own character.

Furthermore, it's possible to counter-sacrifice. I've had this happen more than once, in fact, when two characters have a thing going and they are stuck in an oh, so tragic situation where one just has to die. So one player declares sacrifice only have the other one immediately counter-sacrifice. This is all good as far as I'm concerned, as it's never devolved into some highly theoretical "endless loop". If the players refused to make a choice I guess I'd allow the current active player (or the first one in the action order) the privilege of last sacrifice, but as I've never had to rule on that and I tried to make the rules compact, I didn't dwell on it. Another solution is to have both die: the rules text does not explify it, but I have no problem with sacrificing without moving anybody, so if both want to die, I'm all for it.