"It was not announced, publicly at least. But he should be on for a few days this week. "
Watanabe was originally scheduled to return March 1st, which got changed to March 17th due to his diagnosis of and recovery from stomach cancer. I think he simply decided that he was ready to return earlier than he expected. I don't think he is in just for a few days, I think he's back. As to the orchestra being completely full without a public announcement? He has a huge international following, and there were many people who missed him the first time around. A few private calls here and there can fill a house pretty quickly for someone like him.
"Yea Daniel Dae Kim and Jose Llana are both coming in after Kelli departs. They haven't cast her replacement yet."
I don't know when it was announced (I see you posted in Feb, so you may already know this), but Marin Mazzie is replacing O'Hara in the role of Anna.