(I am answering this Meta question as someone who knows almost nothing about the work that the main site question is about.)
I have discussed this general topic in several previous Meta posts (e.g. here, here, here, and here). As you can see from those discussions, I think that there is, at times, a fundamental misapplication of the two close reasons that are relevant here — namely “Opinion-based” and “Needs details or clarity”.
To my mind, the purpose of these close reasons is to close questions that cannot be properly answered. In the former case this is because the question seeks opinions — the question can thus not be answered objectively, and there is no way to measure the value of the answers. In the latter case this is because we simply do not know what is being asked. Any answer posted may in fact be an answer to the wrong question, and there may not even be a question at all.
Where I think we sometimes run into trouble is in how to define “seeking opinions” and how to define “we don’t know what (if anything) is being asked”.
Taking the second one first, I think the most obvious level is where the question as written is simply incomprehensible (e.g. significant language or grammar issues, or no logical structure), or the post is simply entirely a statement with no question at all to be found.
The less obvious level is where the question is perfectly comprehensible from a linguistic and structural perspective, but makes no sense in the context being discussed. This may be because the questioner has no familiarity with the subject being asked about, or the questioner may be operating under incorrect premises.
In this type of situation we often find questions being closed, or at least voted to close, and often the comments indicate that it’s because the question is incorrect. However, I do not think that that is a correct use of a close vote. It’s not unclear what is being asked, the questioner is just wrong about something. A question may be nonsensical, wrong, or silly, but that doesn’t give us license to say “you obviously can’t mean what you wrote, so therefore we don’t know what you actually are trying to ask”. Such questions should be taken at face value, and answers should be posted (rather than just comments) explaining the incorrect foundation of the question. Of course if a question is based on a premise that can be trivially easily determined to be incorrect, the question may deserve downvotes for being uninteresting or for lacking research.
Returning to the first category, “seeking opinions” can also be subject to dispute. As I argued in some of the linked posts, a question is not seeking opinions just because it is possible to answer it with an opinion or because there is not enough known information to answer the question. Virtually any question can be answered with an opinion, so that is not a useful metric. A question seeks opinions when it is clear from the question that it is seeking opinions. This can be because it explicitly asks for opinions, or because it is formulated in such a way that it can only be answered with opinions (e.g. because it asks something inherently subjective, or uses vaguely defined concepts).
Put another way, any question with a tag about a particular work should carry an implicit introduction of “In the context of the factual reality as described in this work and related materials...” unless otherwise specified.
To take an example: Imagine someone posts the question “What is Harry Potter’s favorite food?” This can certainly be answered with an opinion by any answerer saying what they want his favorite food to be. But it should be self-understood that the questioner is not asking what you want Harry Potter’s favorite food to be; rather the questioner is asking about what Harry Potter’s favorite food factually is in the story in which the character appears. Such a question could be answered without any opinions, such as by finding that the book discusses characters’ favorite foods, or that the author has spoken about this topic. If there is no information about favorite foods then an answer can either present the argument that there is no information, or attempt to figure it out based on whatever clues/soft evidence there may be.
Essentially, then, there should be very few questions closed with this reason, as it is not very common for someone to explicitly ask for opinions or to ask a question that cannot be answered factually.
Now to apply this to the specific question at hand, let us examine what was asked in the main part if the post:
As we know the Elves are afflicted with sea-longing, which must have been known to the orc overlords. Why didn't the orcs just bide their time and perhaps even help the elves depart? They could have saved some resources this way and this could allow them to focus on other matters.
We can break this down into several components, and see if any of them run afoul of either of the potential close reasons. We begin with:
As we know the Elves are afflicted with sea-longing,
This is a factual claim about characters in the story. As mentioned, I have very little knowledge of this particular story, so I have no idea if this claim is accurate. However, it should make no difference if it is accurate or not. If it is then it is a useful foundation for the rest of the question, and if it is not then an answer should just point out that the question is based on an untrue premise.
The next snippet is the same:
which must have been known to the orc overlords.
Once again this is a mere factual claim, which is either correct and therefore helps lay the foundation of the question, or incorrect and therefore provides a starting point for an answer.
Then we have the beginning of the actual question:
Why didn't the orcs just bide their time and perhaps even help the elves depart?
While this may not directly follow from the two earlier premises (I wouldn’t know since I am not familiar with the work) or may be wholly unjustified in the broader context (again, I wouldn’t know) there is nothing particularly unclear or subjective about what is being asked. Of course, a reader who doesn’t know what an orc is might find the question unclear, but a reader’s lack of basic background knowledge is not a flaw in the question — if you don’t know what an orc is then you probably shouldn’t be posting an answer. Similarly, though the question doesn’t specify how orcs would help elves depart, or why they would want elves to depart, this doesn’t make it unclear or subjective. It could either be that these details should be basic knowledge to those familiar with the work (once again I wouldn’t know) or it could mean that the question is unjustified, which is not the same as being unclear or unobjective, and would be the basis of an answer.
Then we come to the final part of the question:
They could have saved some resources this way and this could allow them to focus on other matters.
This consists of another set of factual claims, and is thus subject to the qualifications described above. Again, the “resources” and the “other matters” are not specified, but this would also be either basic background knowledge or an opening for an answer to refute the claims.
In short, the question seems to be a straightforward inquiry about character motivation, and can be answered as such. It can be clear what a question is asking even if it is not clear why the question is being asked. Questions shouldn’t be closed for not being compelling, but for not being answerable.