I'm glad you're enjoying the book!
I'm not sure about getting your stamps in your journal. I would think it would be better to have an official credential, since it'll make it easier for the people who give you your Compostela to look through. I've heard they look a lot more closely if you've walked a relatively short distance. I've also heard it's best to get two stamps a day if you're starting so close to Santiago. Maybe someone else will be able to comment on that.
Some of the refugios have credentials available--though I don't know about the one in O Cebreiro. You can usually also get one from your home pilgrimage association (the Canadian one mailed me a credential in exchange for a $5 donation).
As far as your bag goes, you just have to leave it in the dorm. I always kept my passport/credit card/etc and my camera with me, and never had any problems. Neither did anyone else I knew. Theft does happen on the Camino, from what I've heard, but I don't think it's all that common.
Anna-Marie