Yes currently julius is the only person who can use time magic.
Also in regards to why langris is a magic knight, remember that being one is a big honor. He's a very prideful person too so being able to be one and also be in the top squad is a big accomplishment for him and it's something he can brag about showboat over. So if anything he joined because of the status and less so because he actually wants to help people.
Finally I don't think julius can just ban people from joining because of their ideals. I think his whole thing is as long as you can protect the people of this kingdom and work together with your comrades then that's good enough for now. We do know though that he want's all of his magic knights to see each other as comrades in arms and not act like one is better than other but you know just like real life you can't force your ideals on people. Especially with the nobles who have had corrupted ideals for a long time.