Note that I am a bit wary of item creation rules due to what I feel are a set of loopholes that are far too easy to abuse. I generally place some strict requirements on item creation when I GM a game.
So I'd say it'd depend on what your GM plans, but while there is some downtime between books it's in the order of days at most and I don't think it's ever stated to be more than a week or two assumed anywhere. It is a fairly fast paced path compared to ones like Kingmaker or Jade Regent where there are weeks to months of downtime worked into the adventures. So s brew potion and the spell casting item feats (scroll,wand,staff,rod) I'd say yes. Craft wondrous item maybe depending on the type of item you wanted to make. But I think it'd be difficult to justify craft magic arms and armor.
It's really so simple for the GM to just say the items were crafted in a week or two, or however long the party has between adventures. I would just discuss it with the GM. If he's a stickler for rules, and will calculate exactly how many days it will take, knowing you just don't have the time because of the adventure, then it's probably not worth it. If, on the other hand, he'll progress the story, regardless of how long something should take to craft, then it's worth having the discussion. It's definitely a conversation you should have before creating a character and starting the AP though.
The Geas does allow for item creation as long as it's part of efforts to continue toward Baba Yaga. Or in other words, this enchantment of Baba Yaga is Mythic in nature, nearly sentient, and able to comprehend that just mindlessly marching to a goal would in fact keep the goal from being realized.
That said, it's your campaign. You can ban the Item Creation Feats if you so desire.
The Geas does allow for item creation as long as it's part of efforts to continue toward Baba Yaga.
I'm not sure the Geas is what JoeOutside was referring to when he mentioned "circumstances". Consider the situations the party finds themselves after level 3, the earliest they could take an item crafting feat:
Taken together, there are some opportunities to craft items, mostly the times the PCs spend within the Dancing Hut. That said, I feel that this AP is meant to be fast-paced. Including lots of downtime would tend to slow things down.
Same problem my group ran into around the middle of Jade Regent. And we have a dedicated crafter. I'd suggest using the Hut and any caravan rules you want to handle 'making while moving'. Buying a wagon is not a bad idea outside of the hut.
? How? I mean...
EDIT: Spoiled the spoilers. For those that didn't click it, I basically asked how terrain was going to affect a wagon.