Now regarding the Champions League schedule with a new format, it has actually been discussed for a long time. However, we are all in this new format, so clubs and players must enjoy it even though it cannot be denied that a denser schedule will drain more energy too.
I don't think that all the players would be happy when selected to play for their country this season since the game will be stressful for them, and I'm confident that if a player plays every game this season, they will go down due to injury because there are about 70 games in a single season. Many players will miss these international games because of injuries, and I'm not sure whether all of them suffered serious injuries. Perhaps they are simply avoiding the international games for the moment, so they can concentrate on the league games after.
Now Dortmund is in 7th place in the standings and if you look at their performance I am pessimistic that Dortmund can reach or finish in the top 4 this season.
You also observe that Dortmund's performance appears to be strongest at home, but it goes down when they play away. Dortmund has lost to Augsburg this season and their last game before the international break to Mainz, a team Leipzig has defeated away this season. Dortmund's performance cannot be relied on at this time; despite the fact that the season is still early, they must perform much better if they are to finish among the top four this season, since their current performance will not get them there.