Well i agree of with, i think your a man with brains. People are just different, one have a different way to believe, some people need alot proofs other not, depends alot.

Im not sure about the time travel problem, they (visitors) and i dont mean only Aliens, lol thats a half of it, but actual humans from the future, traveling to now, to fix some things.. like, changing the path of something, that will avoid a World war 3, and if its isn't happened,(now a little of speculation:) maybe its because they fix it, because, the survivurs got alot of mutation, from DNA, that in some centuries, it will cause a massive genetic problem, caused by a NUke, some centuries ago ( which is now) so they just came, to fix it, to change their own future, our future.

And time travel, its a thing that is possible, with portals, its something now yet very well known.
The other way, is... Jump rooms right? like jump rooms to mars, perhaps.
Anyway i must say it. NASA, officially published, a article about..what they found, a sort of portal on the earth atmosphere. Which can teleport matter, sun injectio matter, which they studies the portal dimensions, etc etc.. Which opens every 7 minutes etc etc.. So its possible, it exists, and we can use, it, we just didn't know it.

So most the problems of aliens you stated, will be neutral. and if they are more advanced, why not consider it as a probability.

Alot of same, and only us on earth, well like on the movie , than it would be a wast of space.