Overview of the Problem
There has been quite a lively discussion on the particular query about the spaceship and whether the limitations placed on the spacecraft are "world based" or "story based". I think the discussion applies generally to the other queries cited, and also to questions here in Worldbuilding generally.
Let us remind ourselves what WB.SE's mission is: Worldbuilding Stack Exchange is a question and answer site for writers, artists and others using science, geography and culture to construct imaginary worlds and settings. We impose upon our members certain limitations: to ask queries about specific issues with worldbuilding and we ask them to refrain from asking anything not directly related to worldbuilding.
Let us remind ourselves what worldbuilding is, within the scope of our community: World building includes geography, culture and creatures for the world, not to mention magic and planetary physics, in short, everything from the physics underlying your reality to the entire universe you want to build.
Specifically to the point, Worldbuilding is not for questions relating to plot and character building (as they) are out of scope for the site.
The first query boils down to Narrative Necessity. The author has imposed some arbitrary stricture on a plot element (in this case, the spacecraft) and is now asking us to justify this plot element. Ultimately, the answer to why the spacecraft can only carry eight people is "because the author determined that the spacecraft can only carry eight people". The author could have similarly chosen a different arbitrary number like 6 or 11 1/2.
I hold that this query is very clearly "story based" for the following reason: the stricture is imposed by the story's author / querent and not by any force of nature or other system or rules by which the universe operates. There is no natural law at play within the context of the query that can allow for the variant reading of "world based". Furthermore, the title & the body of the question itself have nothing to do with the nature of the fictional world or the systems that make it operational. The topic and focus relate solely to matters of narrative necessity.
Conclusion: we don't justify plot element questions. This is a classic, *story based" query.
The second question also boils down to Narrative Necessity. The author has imposed some arbitrary stricture on a plot element (in this case, the number of magical healers & their personalities) and is now asking us to justify the situation. Ultimately, the answer is "she is your character and she has free time because of the personality qualities you gave her." Again, if the stricture is not present, then the narrative potential is lost.
Conclusion: we don't justify characters' personalities, predispositions or likely actions. This is another classic "story based" query.
The third question is problematic for other reasons, which I won't go into here.
In a nutshell
The long and short of it is this: whenever a query's essential focus is on any element of a story that itself is not directly related to worldbuilding, such as a plot element or narrative element or character element we're now dealing with a story based query. When an OP frames her query in terms like "in my story" or "for a story I'm writing" or the like this is a sure sign we're not dealing with the nature of the world itself, its subsystems, natural laws, or any element of worldbuilding proper, but rather we're dealing with narrative threads within that world.
Even though we all love stories, our focus here in WB.SE is to help people make fictional worlds: systems, functionality, nature of reality, evolution of forms, etc. Specific to writers, we help them build the foundations upon which their stories are written; but we have chosen not to help them write their stories.
I'd argue that the first two queries needed to be closed without question. They are clearly story based and not world based. They can not really be salvaged as written as they have nothing to do with the nature or structure of the world itself.