The "unknown" as you put it doesn't exist in the setting until it's written. That's the flaw in your argument.
No, that's the point. If it isn't explicitly negatated by known canon, then it IS possible, because it isn't written yet.
If i told you that Columbus isn't the capitol of Nevada, California or South Carolina, and you took that to be true (canon), then you have no idea where Columbus is, but you could argue taht it's somewhere. According to canon, Columbus could be anywhere, capitol or not, except as capitol of the three states mentioned, because canon DOES NOT state otherwise. Compare that to canon stating that Columbus is the capitol of Ohio, in which case canon is clear. Just because something is not mentioned in canon doesn't mean it doesn't exist, and it would be impossible to logically argue that something not disallowed explicitly in canon can not exist.
Now, it's my understanding that Fallout canon does not explicitly disallow additional Enclave bases outside of the Fallout 2 area. Therefore, Enclave bases outside the Fallout 2 are ARE POSSIBLE, because canon does not disallow such bases.
I'm done with this now. Again, I'd rather see a different antagonist, but Enclave would suit me OK.