The 3rd point, i.e.
Do nothing as my concern is not valid
In fact this statement from you question is already wrong:
if a question can be answered in an existing site, then it belongs in the existing site; as per the Area51 FAQ.
This is stated nowhere in the Area51 FAQ, the only points related to this I could find are
In general, if a site makes sense as part of a bigger site, it's better to have one big site than a bunch of little niche sites. Site X should be subsumed by site Y if:
- Almost all X questions are on-topic for site Y
- If Y already exists, it already has a tag for X, and nobody is complaining
- You're not creating such a big group that you don't have enough experts to answer all possible questions
- There's a high probability that users of site Y would enjoy seeing the occasional question about X
But all those points only apply if the whole topic of the beta site would be on-topic here, which is clearly not the case, as questions about Science Fiction movies make up for only a small part of all the questions on Movies & TV.
So as discussed already in the relevant meta sites it is entirely upto each and every user himself where he asks a question about a Science Fiction or Fantasy movie if it is on-topic on both sites, as long as the same user doesn't deliberately cross-post questions.
And as to the genre tags on Movies & TV, those are really only used for ID-questions and general questions about the genre, not for questions about specific movies, so its state as the "most popular" genre on the site is debatable anyway. But you can have those ID-questions if you want. ;-)