I can't advise on transport from Biarritz, others more knowledgeable than myself will no doubt respond to that part of your question, but if you are starting from SJPDP and the Route Napoleon is open, then the demand for places to stay between SJPDP and Roncesvalles will be shared between that route and the one via Valcarlos. If, on the other hand the mountain route is closed there might be pressure for beds if you break that first stage. My understanding is that there is ample accommodation at Roncesvalles for pilgrims, and that, in any case, most Spanish pilgrims do not start from there. When I walked in 2014, Good Friday coincided with my arrival at Puente La Reina and there was a real scramble for beds in the following days but that pressure eased after the end of Semana Santa. This is only based on my own experience and on reading the comments posted by other forum members, and this is such a wonderful time in the Christian year to be in Spain. Buen Camino