Buen Camino Iriebabel!
Even if you find pilgrims that are willing to split a taxi, you should all consider taking the bus. The last time I bought a bus ticket, the bus was 22 Euro, and the last time I split a cab with 3 others, we still paid about 35 Euro each. You get to SJPPdP about 15-30 minutes faster by taxi (depending how crazy your driver takes you up those mountains), but the views are the same. You can also buy the bus ticket at the ticket office in the station. The bus never gets even close to full during the time of year you're going, and La Conda is the bus company that runs the route, if you need more information check out their website.
As for weather and the Orisson albergues, the Napoleon Route should most likely be open, unless some crazy storm comes from nowhere and shuts down everything. Odds are likely not, but hold on to the small possibility that you may have to go through Valcarlos, but there is nothing wrong with taking the route that the medieval pilgrims traditionally took to Santiago . With all that being said, I still recommend you make a reservation with the Orisson albergues. You will never forget the views and the albergues up there, and they will understand if you have to cancel due to weather. Make the reservation on their website or call them ASAP! They don't have many beds. Lastly, if you are concerned about your knees, have a café au lait and croissant in SJdPP, and leisurely go up that mountain. I've met too many pilgrims that had to cancel their trips because of preventable injuries.