I'll play along :)

Let's say it's aliens then. From the point of the discovery on, we would be monitoring Tabby's very closely, and we'd have loads and loads of people working on learning whatever we can from there, so I think we'd get a pretty good idea of how to approach the whole thing very soon.

Even if, for whatever reason, they were a potential threat, unless they've already been on their way for a while, we have 1.5k years to prepare for their arrival or strike or whatever it is we're afraid of. I think this would lead to a kind of Watchmen situation, if you know what I mean. Sad as it is, a common threat to all of mankind would probably be a really good thing for us, if not the very best thing that could happen, as all the petty internal conflicts would be put aside and we'd focus on saving our asses. And with 1500 year to do so, I personally believe no matter what it is that's coming, we'd manage to outsmart it and be ready for it by then. I mean, look where we were 1500 years ago - and we've been using A LOT of that time messing up with one another for various reasons.

(yes i know global warming is basically a serious threat to mankind and yet we're not very good at unifying and solving the problem, but there are loads of human-against-human types of shit going on around that, which blurs the urgency to some people, and has others actively working against solutions to the problem. I don't see anybody trying to facilitate the extermination/enslavement/whatever of mankind just to make a profit or piss off their ex, though.)

I think you were kind of getting at the fact that some people would probably go batshit and call it a threat long before we have any reason to believe so. I agree with that, and in that case, it might very well be oil industry vs. the planet again, i.e. some people using the concept of a threat to serve their own interests. But even that wouldn't be that bad, I think. Firstly, like I said, I think the second we confirm it's aliens, the hype will be so huge we'll learn A LOT about them quite fast - so if it turns out not to be a threat, the asshats trying to sell us that will have to take their guns, shut up and go home. If it turns out they were right (bleh), well, I guess their being dicks gave us a slight head start, so it's not even that bad?

Secondly, just think of the implications of such a discovery. I think it might have a big enough impact to really change everything about the way we, as societies, think. It might turn out the asshats might not manage to turn it into a money-making machine or whatever their intent was, because there wouldn't be that many people paying attention to them this time. We'd all be looking up and talking about what this means for us and our place in the Universe and all.