Welcome
@Teej41 ,
Sounds like you're already better prepared than I was. To answer your questions;
a) You don't need to go to the pilgrim office but I would recommend it. Not only will you get a lovely sello to signify where you started but they will also give you useful information about the route up to Orisson/Valcaros/Roncesvalles, albergues in SJPdP and a guide to almost all the albergues on the entire CF. And if that weren't enough you'll also get a ziplock bag to keep it all in.
b) You only need a one sello per day until you hit the final 100km after which you will need two.
c) This all depends on how many spaces are in Ivar's credential and how long you think your journey will take. If the worst comes to the worst you can always pick up another along your walk. Some Albergues can be a bit careless with where they mark your credential so if you feel you might run out of space you can always ask them to bunch them up a bit, I did this in order to keep them on ones side of my credential and allow me to frame it with all sellos on display once I got home.
Buen Camino,
Rob.