Gate rental fees go to Stephen

No, not really. AFAIK they go nowhere. They are just subtracted from your cash upon rental and the month changes
No, you cannot rent out gates (and before you ask: this will never be implemented due to possible cheating).
In the past you could only create hubs (or focus cities) on a specific continent (I prefer the word region). With the launch of multiworlds you may be allowed to create a focus city on a different continent. Example:
7 (1)
This means you can create 7 focus cities on the continent you created your first base OR 5 focus cities on the continent you created your first base AND 1 on a different continent. Note that the number 5 is not a typo. A foreign base counts twice.
Where did you get the expression "fluctuating airports" from? (I can't explain it without knowing the context)