You can still log in with a VPN on, but when you leave the incog then you'll automatically delete the cookie signing you back out. If you reject all cookies then you can't log in.
Not sure what all data Newgrounds sells but there is no curating and unless you're donating to a creator then there is no incentive to give them any personal information. They will just have your email, username, and a random IP from somewhere else in the world.
Your Feed on Newgrounds is just posts from people you've followed, nothing more. When you go to the art tab you see what you would see if you were anybody else.