As I find is often the answer to questions people ask around here, the only real rule is the Rule of Funny. Go with whatever works best for you and your story.

It is common that when someone uses plural s instead of posessive, it creates one or more copies of whatever is being discussed.
Likewise, when someone uses possesive instead of plural, you can make something out of it. In the excample you mention it could create "the Pevensie's all". What an "all" is and how it looks, would then be up to you to decide.

However, I don't think that there are any hard and fast rules for what excactly creates and a mini and what makes other kinds of shenanigans. I have seen other writes do it differently and never seen them geting corrected for it. My preferences also has a lot to do with the fact that a) they give room for other kinds of comedy than simply having a mini appear and b) they keep the number of minis down.

Using a common spelling for an uncommon name, would definitely produce a mini. Unless you have a amusing idea for doing something else with the misspelling, in which case the Rule of Funny applies.
In my second mission the author wrote "Wildlings" instead of "Wilders" and rather than making a mini, I chose to have some Wildlings from A Game of Thrones appear. I was up to my eyebrows in minis in that mission, so it was nice to do something else.

For example, in my last mission, the author missed using the correct punctuation and said "Rogers" instead of "Roger's". I recorded this as a mini rather than having multiples of Roger running around.