Jamie did complete his puberty cycle. It occurred on his birthday. The screen was set to enter it final stage at that point. Jamie was also having headaches prior to that. It was the night of his first command performance for the Emperor, upon showering when he returned to the dorms, that he noticed the black smudge that he thought was makeup.
It was the Ghourm DNA that gave the their wings, not the the bird DNA everyone believed.
Though it was brief, I think the small part with Matthew and Lancelot was meant to give us a location to identify them from. The Princes and Scribes were also present at the Ball.
Remember, Jamie set things in place to help him remember things. Damian died for the bracelet. Renaud knew where Charlie was and probably had orders to remind Jamie if certain things hadn't occurred. It's possible, the same thing was set up with Giovanni. Fortunately, it was the Golden Orb that finally let Jamie remember where the Orb of the Unicorn is located, also leading him to Charlie.
The seat of the Prince wasn't filled until the meeting held after the rescue of Jonathon.