1) I really liked the Irrisen book and found it useful to give me some context around the country's culture and history.
2) It was alright, as much as any of them are. In my opinion there's no point playing anything icy as so many monsters have resistance to cold. It makes the Winter Witch a little pointless, but there is good loot for witches so playing a vanilla or other type could work.
As a separate point I'd suggest your party decides what the overall alignment is going to be. This AP really can lend itself to a good, neutral or evil party so they just need to decide what approach they want to take. The group I had were mostly good, so the tone was set in that direction and I made changes to some of the RP elements to reflect that.
Winter Witch is actually not a bad option in this AP, seeing how it gets immunity to cold at level 14 and has a modest amount of cold resistance before that. While you might find the amount of things with cold resistance annoying, most of those things also use cold damage meaning they'll be just as annoyed with you. The Winter Witch prestige class is probably one of the best in the game as it allows you to keep getting abilities from your old class at the levels you would have gotten them had you stayed, and allows you to ignore half a foes cold resistance, and eventually allow half the damage from your spells to ignore cold resistance and immunity à la Flame Strike. Heck, if you take the feat Prestigious Spellcaster you can have all that without sacrificing any spell levels for it.
Really the only issue with it is that you can't cast fire spells, but that isn't as big an issue as it seems because A) The amount of fire spells on the witch's spell list is pretty low anyway (There's more cold spells on the list and witches are more of a Save-or-Suck kind of class anyway, which being WW doesn't interfere with at all), B) nothing is stopping you from summoning creatures that know fire spells, and C) you can always use UMD to if you need fire magic (the archetype never said you can't use Spell Completion Items or things like it), but even if you ignore all of that you'll get bonuses to counterspelling or dispelling cold effects, which as you might imagine are pretty common in this AP.
Add in the thematic connections with the AP Winter Witch is a surprisingly solid option for this AP.
1) Very useful for book #2. Especially the map.
2)
The players Guide - don't know. I wrote my own based upon how I intended to start things.
Ideally (IMO) the players should NOT know that they're about to be whisked off to a land of perpetual cold, witches, & fey. And they should definitely not be prepared to head off to another planet or WWI Russia....
So I'd just skip the PG altogether.
The Winter witch Archtype & PRC being mechanically good/bad/other isn't my concern as the DM.
(My) Winter Witches operate just fine. :)
This is much more important to one of my players though. He started as a druid & ended up needing a replacement character early in book 3.
So he made a Witch (Witch, WW archetype, WW PRC, Jadwiga) who'd been swallowed by the Hut while trying to help Nahzeena sap its' power back in Whitethrone. Of course that's not the story he sold to the rest of the party...
He's not noted for making ineffective characters & he seems happy enough. And he's able to handle the encounters.
He has commented though that it's odd how few cold spells are on the WW list.
I used the Irrisen book a lot for providing extra flavor and context in Whitethrone. I also used Cities of Golarion. I added some extra plotlines with the stilyagi and the theatre, because one of the players is a bard. It provided some memorable encounters.
I also used the cold sisters in a later stadium of this campaign.
We are almost at the end. The group will probably fight Elvanna next Saturday. I am going to add some Irrisen stuff (again from the Irrisen book) and then play Witch war legacy, with some adaptations.
After book 6. Otherwise that adventure will be way too difficult. My group defeated Elvanna last Saturday, so we will be starting next time.
I didn't like the Player's Guide because it revealed a lot of things that some level 1 peasant living in Heldren couldn't possibly know. "You learned this growing up in Irrisen" ... no you didn't.
I wrote up a crunch summary and gave that to my players instead.
I borrowed a copy of Irrisen: Land of Eternal Winter and found it very helpful for book 2. There's nothing in there that you can't make up on the fly, if that isn't available. But it's a pretty fun resource.
I didn't have Cities of Golarion available, and have no idea how much it helps.
As to Winter Witch Archetype and Prestige Class, I'm a huge fan of letting the players play what they want. There are definitely role-playing situations where that will be hugely helpful. Some where your Witch will have to overcome a lot of (utterly justified) prejudice. Some fights will be trivial, some will be impossible. But that's more or less true for any class.