I agree with the above on when it becomes collusion. Just splitting the blinds, and not playing too hard against the other live opponents is theoretically reasonable poker, because you're avoiding the toughest opponent at the table (and generally the biggest stack if the rest are sit-outs). There's obviously nothing wrong with that since it's pretty standard
poker strategy.
Checking down an all-in is similar, in that your goal in the tournament is not so much to amass a lot of chips (though that helps) as to move up the placings, and you do this by eliminating the all-in players. If you think you can only win the hand through
bluffing, then it may be +ve to bet, even though it risks letting the all-in back into the game, and if you've the nut hand then by all means get the money in to maximise your profit, but the in between hands where you don't really know where you stand the
equity from increasing the chance of knocking an opponent out can mean it's just good poker to do so, so long as you don't explicitly agree to do it.
As for what is collusion, it might worth giving some kind of definition. I'd define it as discussing information or strategy with another player to gain a mutual advantage.
Note that by saying mutual advantage you can exclude any heads-up action where you talk about your cards or show a card to induce an action - that's a whole separate discussion. Any thoughts on the definition?
I'm sure it can be improved, but it's one to start with...