#1: In a competition game is cheating. You can only load from a save game when an accident happens or a crash (i.e. C3C). In a fun game you are doing it for learning better play so it's fine. If you do it afetr any lost fight or slightly bad thing then you're only cheating yourself out of a good lesson
#2: It's legal and in the rules. It's been there in various forms since Civ original. When I first showed my friend the tactic in civnet he was both devastated and impressed and upset all at the same time. He's still not recovefred form the facial contortions necessary to portray all of these emotions simultaneously
#3: Never done anything like that since the original civ. Back then you could get a new technology every time you captured an enemy city. Nothing wrong with it but like #1, who are you kidding? It's only fooling yourself into beating a bad AI implementation. I'd do it when I know the AI is better than me but doing it on lower levels is self delusional
I would not call it cheating however and if it was outlawed in competition games I would be surprised.
#4: Never knew this one - I have had armies caught on enemy land after war declared (from a MPP) and my Armies were attacked. Do you mean just the army commander on his own is not attacked? Or the army with followers is not attacked? I would say that it's an explot if an army with no followers is not attacked.
#5: Exploit and bad game mechanics but not cheating as per #3. At the very least, the unsupported units should not disappear all at once. They should convert to various types of units both controlled by the same player/AI and some controlled by Barbarian AI or to population explosions/unhappy citizens. I don't believe the previous poster who said this has happened many times in the past in history. I have never heard of the people making up a HUGE force simply disappearing of the face of the planet
Armies cut off from their support lines may disperse it is true. It's not immediate and even the ones that do "disappear into the countryside" end up somewhere. So that's why I call it bad game mechanics.
I employ a similar tactic to #5 but mine is a bit more "realistic" although not very nice to think about. I will draw out a huge enemy fleet or definsive force and after meeting their challenge at a good spot, withdraw my units to a safe distance then nuke them. It's an honest way to reduce a huge stack. To hell with my score - I only want to beat those buggers! In any case. if I can afford to use nukes I've usually all but won anyhow. I only use them when nobody else can...Some pretty close historical truth in that eh?
Since I like playing conquests too, I usually opt for lethal bombardment instead. Bombers and Artillery are my favourite stack reducers and they don't cost me penalties for using nukes. People that have not played conquests can only imagine the pleasure you get from being able to bomb units to death.