Regarding the injuries, I don't think it really matters whether you are present or not. They just seem a random occurance. There do seem to be seasons where there are more injuries than others though.

As regards your other point. The cup always has a mixture of teams at different levels, judged (at the moment), on quality of teams. Up till two seasons ago, it was common for a low level team to be drawn against a level 14+ team in the early rounds, so its a bit more balanced now. The cup is the only competition where you will be in with different level teams. Usually, when it comes to the league and champs league, 99% of the time, you will be up against teams from your own level.