A1: Payed a lot of money, got the 6* and with all the free stuff we get on DMM periodically, he most likely had more than enough items to upgrade the girls to the character lvl you see on the picture, while the player himself still remained quite low.

A2: No idea what you mean. You can keep those knights you want, and you can delete anything else.
I personally have one of each, from every Iron, Bronze and Silver (not sure if I got all the silvers though), max skill and max item slots, and keep them for collection purpose as well as a manyu eater, when there is no one else to lvl up for daily quest.
And of course the argument can be made that they are of some use (if fully equiped with L10 items that is) for the Nation Battle Scenarios.