Well the way I see it is Ben was created a baby with Glory in him, it just wasn't until recently she started to emerge. Personally I thought she had just started this year (which is why she hasn't been seen in Sunnydale before) but that doesn't really make sense now that I think about the points brought up here.
I don't think Ben is immortal, the head Knight said Glory will live out Ben's mortal life and die with him. I assumed he was born like everyone else and Glory eventually came out, the General guy also said her power could not be contained by Ben so I assume it worked at the start (keeping her in). Although I'm not sure how the Knight knew all this but not the identity of Ben himself. Also I think the Knights were just after the key, that was their purpose and their interest in Glory started when they realized they were after the same thing.
I don't think Ben is immortal, the head Knight said Glory will live out Ben's mortal life and die with him. I assumed he was born like everyone else and Glory eventually came out, the General guy also said her power could not be contained by Ben so I assume it worked at the start (keeping her in). Although I'm not sure how the Knight knew all this but not the identity of Ben himself. Also I think the Knights were just after the key, that was their purpose and their interest in Glory started when they realized they were after the same thing.