I would say no. It's more generous than most campaigns in giving you allies and items that can substitute for spells (especially clerical spells). My winning group consisted of ninja, inquisitor, magus and oracle. The oracle was the only full caster, and I think they could still have succeeded with, say, a bard instead.
No, but they help. As always.
Full caster is not necessary. My group had samurai, ninja and bard who for most of the time were just fine. Sometimes they had Spivey (many levels below PCs) with them as heal bot with amazing number of channel energy but that was all of it. But as magnuskn said, it helps, sometimes alot. My group didn't have any access to travel magic so it slowed them down in last two books significantly (they used almost whole spring and early summer story of Tide of Honor and end of summer and autumn to travel to Kasai) but it was fine to me and the players.