Incorrect. Most of the quests throughout the game are primarily run by characters of around the intended level. That should be obvious if you pay attention while playing DDO.
Favor runs are not a significant factor, because each character only ever needs to get the favor once. There are also some low-level quests with loot that remains attractive later, such as Xorian Cipher: but because dungeons don't need as many players as raids do, that doesn't create an obstacle to the formation of lowbie groups.
Because they impact gameplay for everyone. It is blatantly fallacious to claim that the actions of other people don't matter. Here's a large article explaining why that is.
Incorrect. The claim to which you are referring is an elementary aspect of game design, with which every game designer around the world would fundamentally agree.
If you don't see why that is, there are schools and books that would be happy to help.