As an interjection, they have totally different meanings.
As you say, "まあ!" corresponds to "Oh my!" and is decidedly female. But, "さあ" can be used only in this way (I think):
A: "Shouldn't we start now?
B: "oh, I don't know. . . ."
A: "さあ! (C'mon!)"
In other words, さあ indicates encouragement or invitation. You sometimes hear "さあさあ！", which is just a repetition of the above さあ and its meaning is the same as the single さあ but just with more emphasis.
The other uses you mention aren't interjections. まあまあ is quite different (you can call it a separate word) and as you say it means "so-so". Another まあまあ is
Father (to his daughter): "Get out of my house!"
Mother: "まあまあ, she say she won't do it again, so forgive her this time."
I'm not sure if we should count this as an interjection. We could say it is a fixed phrase to mean "calm down" or "be lenient/patient" or both at the same time.
On the other hand, look at this this conversation:
A: "Do you know which team won yesterday?"
B: "さあ . . . (I'm not sure)"
This one is again quite different from the interjection さあ。You can think of this さあ as a fixed phrase meaning "I'm not sure".
Another possibility is the suffix(?) さあ as in "昨日さあ". This さあ is used to add emphasis on the preceding word and it doesn't have a particular meaning itself.
I'm not sure if I have covered all possibilities, but off the top of my head, I think I have.
