There is a bus at the airport in Biarritz to Bayonne; once in Bayonne, you can go by train or bus to SJPP. (Unless the trains aren't running, then you've got the choice of bus, bus or bus. Or walking! Start your French portion of the Camino sooner than you thought!)
SJPP and Roncesvalles are small places. If you want, you can reserve a B&B or hotel or bunk in a private hostel/gite/alberque in SJPP, or you can wait until you get there, go to the Pilgrims' Office to check in and ask the lovely person helping you out to recommend or call an alberque for you. I think there are one or two hotels in Roncesvalles and one great big alberque. You can call to reserve the hotels but you can't make a reservation at the alberque...from my understanding. Someone will correct me if I'm wrong.
Differences? Oh yes, m'dear. A whole country of differences! You just wait. You'll walk with a big smile on your face as you see how beautiful the Basque region is, how lovely the Meseta is, and will be reminded how wonderful Galicia is!
OK, here are a couple for you... completely insignificant:
Difference: in Basque, if you ask for a Cola Coa, you will get a big hunk of bread to dip in your hot chocolate. (Nom nom nom!) In Galicia, if you ask for a Cola Coa, you will get the Cola Coa. (Still nom nom nom!)
Difference: The Basque region is green and brown and sometimes wet (raining), sometimes dry (not raining), sometimes horizontal (windy), sometimes flat (not windy). Galicia is GREEN and brown (the soil) and sometimes wet (often, rainy), sometimes dry (maybe), sometimes... Well, you get the idea!
Don't be freaked out, Jason; be so excited that you jump up and down like a little kid! You are going to back to walk the Camino again! Lucky you!!!! Buen Camino!