Yeah, if you arrive late in SJPdP you should definitely have made a reservation somewhere. As said before, it's not a very big town, and only about a 15 minute walk down from the train station. You can't miss it, and the whole town is close to the Camino...you can't miss that either.
I wouldn't sweat the credential thing. I picked mine up from the pilgrim's office there on both my Caminos, which were during the peak season. Neither time was there a queue (line) to get the credentials, and it took no time at all. Maybe ten minutes. To me picking up the credential there and registering kinda seems like all part of the experience. Besides, I like the credential they give you. It's made with nice thick paper, and they stamp it. Really nice people running the place.
Believe me there is nothing to panic about when walking the Camino. Piece of cake.