So my thoughts on this are as follows:
I see no issue with asking questions on how to obtain an unobtainable item. In this example, after an update when someone sees a new item they like and want to know how to get it. There is no guarantee that someone checks every fansite or that the information on those fansites is accurate. They may just think someone hasn't updated the info yet and want to know for themselves. Answering the question with "This item is currently unobtainable." is a perfectly valid answer. I know it is harder to prove when something is false but sometimes the correct answer is "No, it can't be done." or "No, you can't obtain it.".
There is a similar question How to obtain Gilded Eldritch equipment? that has no accepted answer yet. The only answer is a YouTube video pointing to someone dropping a piece of equipment. It can be assumed they are all dropped by The Brainstealer but looking at Wiki all that information isn't entered for those items yet.
As for someone creating a question for every item on Wiki's unobtainable list I don't believe that would be against the rules of TibiaQA exactly but should be frowned upon if someone is just spamming questions that they seem to know the answer to already to farm points (though creating and answering your own question is valid for sharing knowledge). Also, TibiaQA and Wiki are separate fansites. The rule of TibiaQA in What questions should be avoided? is "Easily answered by official manual". There is nothing mentioned about info on other fansites. Someone could be new to Tibia and this is the first fansite they are taking advantage of. It is common knowledge for most Tibians that Soft Boots come from the PoI quest. Asking "How to get Soft Boots" is a perfectly valid question though for someone to ask on TibiaQA.
As for people accepting incorrect answers on unobtainable items that is the nature of TibiaQA. That is more an issue with people posting low quality answers. You are asking the community to provide you with information that you generally do not know yourself and are hoping it is accurate. If you ask "What is the best damage type to use when hunting Dragons?" and someone answers "Fire", that is not the questions fault. Anyone providing an answer should do their best to make sure the information they are providing is accurate. Using at least one source is always helpful, multiple sources can be better to confirm the information. TibiaWiki is an excellent resource which is why I use it as a source for a lot of my answers even when I already know my answer is correct. Providing a source just adds credibility to an answer. The issue with that is not all fansites are as accurate with information as Wiki. Especially after an update many fansites can have incorrect details based on assumptions or things that may have changed since Test Server.
With the point of incorrect answers being chosen as best answers. I know I have had a similar conversation with someone a while ago about this around old questions with no accepted answer that have correct answers supplied. Maybe there should be a tool to allow moderators to mark an answer as the accepted answer if it receives enough upvotes, the question is old enough, or some other metric.
I agree this can definitely be an issue if someone is abusing this feature. A user should probably not be allowed to remove a flag on their own question.
As for this, hopefully people would be using the voting system reliably to promote the correct content. The one downside with a voting system is occasionally people will use it to down vote someone because they disagree with them or they don't like them. There is nothing stopping a user from downvoting good content as it is opinion based. Hopefully this wouldn't become a larger issue but if the same users are regularly doing it maybe some action could be taken?