Christianity.SE gets around this problem by requiring that questions be answerable as objective facts, not as beliefs.
E.g. "Gibet"'s example question (Was Jesus a man, a demi-god, or God?) would be asked as several questions, such as:
- Which denominations believe that Jesus was God?
- Do Jehovah's Witnesses believe that Jesus was divine, but not fully God?
- Are there Christians that follow Jesus's teachings but believe he was only a human prophet?
All questions are presented as if asking a knowledgeable atheist scholar.
It's too late to impose that rule here though.
"in-universe" is a generally understood term, and can easily be looked up (e.g.
What does "in-universe" mean? - Science Fiction & Fantasy Meta Stack Exchange).
Its use here wouldn't be inappropriate, especially in comments.
It would be acceptable in questions too, though it would still be better to ask questions objectively:
- What attributes of God-X made the Romans believe he was more powerful than God-Y?
- What caused God-Z to be held in higher esteem in Roman mythology than in Greek?
The first is in-universe (it could be asked of a Roman or Greek priest, and they would understand and be able to answer).
The second isn't in-universe (the priest would be confused by the "mythology" term).