Since both the 'Login' and the 'Create an account' links on the site point to the legit poloniex.com links I think this site belongs to the legit poloniex exactly to avoid the domain to be used for phising.
I hope so, but I would guess it says invalid, then returns you to the normal page, even if it's correct, just to gather data. You would then try logging in again and be logged in fine, thinking nothing is wrong
No. When you hover over the Login link it points to
https://poloniex.com/login which is the legit link (notice the .com not .co) and poloniex.co/login doesn't exist. In fact, not even
https://poloniex.co exists, only the non-SSL version does.
So whatever you click on the site you're going to end up on the legit site. Granted, it might change in the future but as it stands it seems to be harmless.