I rode this route off road this year and did the following
Flew from Gatwick with easyjet as they are cheaper than ryanair for both bikes and tickets
We had a hotel booked in Seville, we locked the bikes up in their garage, they were cable locked to each other. Three bikes, three locks, we were never worried as you could not get into the garage unless you were a guest
Flying back, easyjet do not fly out of Santiago, and as we wanted to see some of the places that we had ridden through, we hired a van and drove to madrid over two days.
As for a bike bag, don't bother, just go to your local bike shop and ask for a box, and once in Santiago, there is a bike shop that will do this for you as well.
If you are riding over the easter periiod, albergues can be tough to get into. We did not stay in one until easter had finished. I was amazed as I had walked the Portuguese and il Primitivo during the same period. Once easter was over, there was nobody on the tracks
Enjoy, it is a great secluded route if you follow the track. It is not to rough, and the historical sites that you will see are amazing