If the earth were to be visited by real extraterrestrials from a galaxy far, far away, then one would reasonably expect that the bible, and "God" in "his" "sovereignty" and "foreknowledge", to mention such a momentous occasion.
If aliens do show up, you can bet your bippy that suddenly people will find scriptures that foretold this would happen. I bet you could find a couple candidates yourself by flipping around the Bible.
The Bible indicates that the whole creation groans and travails under the weight of sin (Romans 8:18–22). The effect of the "curse" following Adam’s "fall" was universal. Otherwise what would be the point of god destroying this whole creation to make way for a new heavens and earth, (2 Peter 3:13), (Revelation 21:1)
Therefore, any ETs living elsewhere would have been affected by the Adamic "curse" through no fault of their own, they would not have inherited Adam’s sin nature.
You answered your own objection here. People always bring up the issue of atonement but it has nothing to do with aliens. If aliens didn't sin, then they don't need a savior. If they did also sin, then they can also be redeemed by (alien) Jesus. Why would this not be a tenable explanation by religionists?
However, Christ died once for all (Romans 6:10, 1 Peter 3:18) on the earth. He is not going to be crucified and resurrected again on other planets (Hebrews 9:26).
All the scriptures that pertain to this subject can be interpreted to refer to this planet only, the same way that the Society constantly explains how scriptures were written to be understood from man's perspective. There's nothing in there about aliens because man doesn't need to know that. So the statement about Jesus dying 'once for all' is in reference to humankind; he is never going to sacrifice himself again, so everybody aboard, this is your one chance.
Additionally, you rule out the possibility that, after seeing various alien races created and loyally serving God, Satan chose to make his move on the newly-created earthlings, and so Earth is where he makes his stand and falls, thus settling the issue once and for all. He didn't tempt any other races successfully, so they will never sin and will not ever need a redeemer.
See? I fixed all the problems. Also, when people say stuff like "Religion will fall if aliens call", their Western bias always shines through. What does Buddhism care about aliens? Shintoism? Hinduism?