A few lines of text here-and-there in the adventure tells us that Winterpocolypse has lasted two years. Everything else in the adventure implies that it has not. Personally, I chose to go with the parts that make it not stupid as opposed to focussing on the parts that make it yes-stupid.
I don't think participants in this thread have been arguing that the error is impossible to correct. I understand the debate to be more along the line of "it's infuriating that in a commercial products glaring and easily to correct errors slip through" vs "the product wouldn't be improved if these errors were fixed, so stop complaining as it's easy to correct". I'd add that if a problem which makes the setup stupid (to quote your own word) is easy to correct, it's even more a reason to be irritated that the problem stayed in the final version.
I mean, I can't even get my head around how a spell would stop the sun from rising in an isolated part of the world, but the rest of the world is fine. (Really powerful illusion, maybe?)
IMHO, that's the part where suspension of disbelief kicks in: one has to accept the supernatural elements of the story to make it work. We are told that Auril does that, and that's magic, so it works [and yes, I'd say the easiest way would be to cover the dale in a dome of light-reflecting illusion if we were to discuss that). On the other hand, if the module doesn't provide explanations for a few things like "where has the food for wild animal come from during the past two years ?", you can't suspend disbelief at the magical or supernatural explanation, since there is no explanation. Suspension of disbelief isn't acception of internal inconsistency. I guess few would have a problem with magic user being drafted into mass producing food to ease the situation for the starved population, as suspension of disbelief requires that the audience to accept the supernatural elements of the story for it to work, if it was a situation described in the book. It is not, however, so we don't have anything to suspend disbelief about...
I've never once in 35 years used an adventure without spotting something that I thought was stupid and ignoring it. I didn't even consciously notice this one until I got involved in this discussion (so quick was I to ignore anything that tried to tell me that it was Full Dark Winter for Two Whole Years).
Like some people here have mentioned, that was probably the result of multiple hands creating this book for multiple purposes. It's an Icewind Dale Gazeteer and a Horror-Inspired Adventure, but clearly there's some tape still showing from where it got pasted together. (And it seems they had some push-back, or back-and-forth on just how much horror to include).
Yes, that's certainly the crux of the problem. The products sits between two extremes. In a sidebar they warn that people might be inconfortable with certain themes. But the adventure downplays so much of the "horror elements" that I don't find anything to be worried about (except maybe the human sacrifices, but in a game where killing people is the expect standard behaviour to solve problem, I'd say it's much less shocking).
The question of "how to solve the situation where the writer couldn't decide between a grim and gritty adventure and a Dale sourcebook?" can be either solved, as you did, by removing the elements that makes the adventure
yes-silly, so you have a shorter winter, a minimal loss of light, and a legitimate worry that the Ten-Town won't survive if spring doesn't occur soon, which is a great adventure setup but minimally horror-themed or you could accept the premise of two years under the evil grasp of Auril, and rewrite many parts to take it to its logical conclusion (downplaying it only as to allow for a few survivor here and there, so there are still NPCs to interact with...) It is
extremely stimulating (and as a DM I'd probably just do that, provided the group I play with is OK with a real grim story) but it requires more work to adhere to the "horror story" of the initial pitch.
With regard to tree surviving or not, the question is "how many trees were there in the first place ?" Initially, I was envisioning a forest-covered area in the dale, like a Canadian forest or a taiga. Reading the module, I developed a diferent vision: even when it's business as usual, there are hints at much less trees than that. There are very few trees on the IWD map, and I think we should read it as "a few small trees and shrubs where there is a tree picture, nothing where there is none". We're explicitely told that IWD has few trees, and 7 out of 10 of the Ten-Towns rely on whale oil for heating because wood is too precious to burn. Even the inuit way of life in the tundra let them have access to wood for camp fires... so the regular situation seems to be one of a much harsher conditions even before Auril's involvement, with only a few shrubs and the occasional dwarf tree.
