League-of-legends – What non-lane minion units are considered “minions”

There is a hotkey H which is the equivalent of toggling "Stop", which stops your current action. I'm not sure whether this will do what you need in stopping the auto-attack, but it's worth a try. I don't see any other options that could remotely do what you desire, other than getting stunned.

Continually pacing back and forth within range is a preferable course of action once you learn how to do it without getting exhausted. This actually has a few benefits besides just preventing you from attacking - it makes you a harder target, and it also makes it less detectable to be seen creeping closer and closer towards your opponent to make a pot shot. Alternatively, it makes your opponent uneasy since you can easily be in range to take said shot, which makes them less inclined to stay close.

Sadly, there's no sure-fire way of resolving this situation.

You won't be able to rely on minions to take out their towers and nexus, especially if their uber status gets to the point where they can start defending their respawning inhibitors. If you get really lucky and get their nexus towers down, you could just farm minions hard enough for yours to take the nexus down a bit then try to dive the nexus. I'd consider this a Hail Mary more than anything else, though.

If you know you're going up against a team that is going to steamroll you once they get maxxed out (2+ carries), the main goal is to take advantage of the fact that they will be weaker in the early game. If you zone the carries and/or kill them enough to keep them from getting geared and leveled, your team will get powerful enough to take their nexus before they can do anything about it.

Assuming you find yourself in that unfortunate position anyway and forcing their team to split up isn't working, you absolutely have to focus the primary damage dealers down first. If they stick together and you're not killing their carries first, you'll take way more damage than you will deal out over the course of the fight and you're sure to lose.