#1 totally agree
#2 totally agree
#3 quite possible. But I gotta believe that they were built and inhabited by human beings using terrestrial technology.
I look at what is possible vs what is most likely or probable. It's possible that aliens from off planet have visited here and helped cave people build paradise until they all decided one day to evacuate the planet and not leave anything behind except for rocks. Nothing like a platinum bar or a stainless steel machine part or anything that could be considered a plastic/polymer of any sort. Nothing that can't be explained by manpower, diligence, and natural forces. Most of the "unexplained" megalithic building phenomena can be explained by a number of interlocking factors.
1. Initial observations are often wrong or incomplete.
2. Basing scientific conclusions on the observations of people in a pre-scientific world has its own difficulties.
3. Throw in observational bias and disdain based on religious motivation - i.e. those heathens can't possibly be bright enough to build something grand because they don't believe in Yahweh, Jesus or Mohammed, so it has to be aliens...
4. Coincidence is mistaken as corroboration.
I know there are "unexplained" structures and other seeming evidence for alien intervention in our world, but I feel sure that as more understanding and knowledge is attained every one will be explained as having terrestrial origins. Show me a leftover alien gum wrapper or socket wrench or iPhone and I'll believe. Until then I have to believe that my human forefathers were just as bright as we are, they looked at the world in a different way, and they had different motivations and resources to use to reshape nature to their own use.