The "accueil jacquaire" (for Compostela pilgrims) in Lourdes is at 16, Boulevard de la Grotte.
Here in Google Maps. It is the main street, full of shops, that goes to the sanctuary.
In France, I always call in advance. Sometimes albergues (gîtes) are small, just a few beds.
On the other side, I feel that the good priest in Arudy don't manage "reservations", you just announce your arrival beforehand.
The municipal in Mauleon does not take reservations, it is as in the Frances, first come, first served. I recommend that if you plan to arrive late, make a reservation in a hostal.
If you have problems with your French, you may ask the kind guy in the Lourdes "accueil" to call for you, and in next stages ask hospitaleros to do the same.
Having a working cell phone is advisable. And carry some snacks, there are not many bars or restaurants in the middle of stages.
It is a great camino, if you want some solitude and magnificent landscapes. There are some threads about the Piedmont in the forum, use the "search" function.