Is throwing a game against the rules?

Yea let me ask our commish. Brb

Yes.

it depends. If he was targeting a specific postseason opponent then I don't see what the problem is. If I know that losing would give me an opponent that has an average team, why would I not lose on purpose?

Now if he and the guy that made it in your spot had some kind of agreement then yeah it's a problem but you'd have to prove it.

My opponent last week benched his QB and defense so that he wouldn't face me until the finals in the playoffs

No but if I can lose to avoid the guy with the highest scoring is like 7-6 and barely made the playoffs,to get guy on the lower scoring total (and average especially as of late, to account for in season moves) then I am doing it.
Doesn't guarantee me a win of course, but you put yourself in the better statistical position to win.

In my league you cannot have empty slots unless you can justify. For example you have a 2 point lead and sit your defense so you don't get a negative score. You can't sit to screw over others. However if you are willing to drop players to pick up scrubs to roster its not against our rules. We've had people toss a game so they would be seeded against an easier opponent. I have no issue with that.