I think at the heart of this debate is whether questions on potentially unanswerable topics add value to the History Stack Exchange.
On the one hand the question itself might be historically incoherent to those in the know.
For instance: "Which German composer invented the piano?"
Such questions can only be answered by critiquing the question directly. An exhausting and sometimes fruitless exercise as you may not be able to clean them up even with the OP's input; as you end up semantically replacing both the handle and the head of the axe - so is it even the same axe?
On the other hand the question might coherent but unanswerable with the knowledge and resources at the disposal of the current stack exchange membership. Perhaps - in so far as we possess the arrogance to speak for all historians - not answerable by any historian with current sources, tools and absence of a time machine.
For instance: "How much of the annual budget of Pharaoh Khufu's regime was allocated to tomb building?"
We can answer these as unanswerable or close them as unanswerable; but we run up against problems either way1. If we answer them as unanswerable, then the first person to post gets votes for proving a negative; hardly fair, and hardly pleasing to most OPs and people accustomed to negation being in comments only. If we close them as unanswerable; we don't allow for someone knowing the answer, partial answers, duplicate unanswerables with the better content2, or incentivising people to explain authoritatively why it's unanswerable3.
Then you have what I call the "zero or many" question:answer paradox. These are questions where a quantifiable answer exists but it's either zero or many answers.
For instance: "Was there a king that had seven legitimate sons?"
It's a paradox in so far that instead providing 0 (no answer - implicit), 1 (no answer - explicit) or N answers (subset of known or knowable answers), the question is closed as too broad or opinion based because it didn't resolve to precisely one answer. When in practice, if a question has multiple valid answers then the question should have multiple answers4; not none to frustration of the OP.
So. To return to the basic criteria of: Does it add value?
- If the question isn't historically coherent and you can't fix it: Close
- If question is unanswerable and the OP wants the question addressed as-is5 and this is the first time we've seen this question: Answer why the question is unanswerable at the current time. Should include the Why so as not to cover for personal ignorance.
- If the question is unanswerable and we've see it before, more or less as worded: Close with appropriate duplicate link
- If the question, if answerable, allows for multiple equally valid answers: Allow any answer and applicable voting on quality of said answers until the high threshold is met for putting the question on hold / freezing out extra answers.
1. Courtesy of Stack Exchange's origin as a computing programming Q&A with clear falsifiability.
2. Unless you didn't close the first instance of an unanswerable question.
3. Comments don't accrue reputation; can not be cleaned or refined after 5 minutes; and not guaranteed to be persistent in the underlying database.
4. Rare cases of a super-abundance of answers can be treated the same as other stacks do through use of the On Hold action for curtailing Me Too! answers.
5. Occasionally the OP meant to ask a question that is answerable, but more often the OP really does want some reply to the question as asked. For knowing what we don't know is vital in any branch of human knowledge.