Have you looked at your map? The cabin has quest marker once you've talked to the NPC at beginning of Luculla forest...
Playing co-op with a friend neither of us ever had a map marker for it. We just kind of eventually stumbled upon it. The mushrooms gave a vague answer as to where to find the scroll and I figured we'd find it doing some other quest just as you had to go through other places to continue the main story in act 1.
That's maybe a bug I guess. But anyway, the cabin is not really hard to find and the NPCs guide you the way...
OF course you COULD do other stuff first. But in the end, you'll see that enemies are too strong for your party or not balanced to your party elsewhere. So following the White Witch quest just seems to be the next reasonable step...
The level 12 enemies were considerably easier than I expected them to be and made me think "Well maybe they just jump 2 levels for no reason?"
Well, at least when you meet the level 14 spiders in the north you should think about doing something else or going somewhere else.
