Knucks,
I like the way you think and your willingness to share your theories.
The Earth is 4.5 billion yrs old and the universe is about 13 billion, so someone has a big headstart on us.
There have been to many UFO stories, throughout recorded history, to believe it is not possible that they exist.
I would think it is more probable that they have occassionally meddeled with species development though lifes history on Earth. Evolutionary theory is confirmed enough, but who is too say they may not have filled in the gaps. Something more along the line of 2001 space oddesey.
Moving species to different planets is an interesting concept and could account for some abduction stories.
As far as government concealment vs religious beliefs, rest assure that the first thing asked would be who created the aliens? It would just create a new religion.
It is fun and stimulating to consider new theories, but I am a firm believer that we are not alone in the universe.
Thanks for sharing your theories