Let's start with what I believe to be physical evidence.
To begin, the background is different. Yes, it can be argued that this is due to a lower quality image being used for printing and other reasons, and that may very well be the case. However, we now know what the blank background looks like. I personally do not believe that the background would be changed for this purpose. I think the background was created first and that it hasn't been touched since. I've heard multiple conflicting points on this, and would enjoy being correct if I am mistaken.
As Artsi Omni has shown, the images shown can not only be created, but also the assets already available can be moved.
It was initially believed Eric to be employed by Marina, a company publicly known to work with both Namco Bandai and Universal. However, upon learning that he is no longer employed there, we do not know the same of his current company. That said, we have not received any statements from his current company invalidating the leak.
The promo artwork for the Grinch movie, I believe, has been combined with different custom assets to give the illusion of being legitimate. The promo Grinch artwork has also been confirmed to have existed beforehand.
Now for that which isn't evidence but is opinion.
I agree that it does take a lot into consideration for somebody to have produced such a paper trail, but it appears as though just a little bit of everything is sprinkled here. It comes across as purposeful to me. It is definitely a longshot story. I'm a history major and while in university, my first professor opened his first class by saying "Nothing in history has ever happened until it did." I think it fits here if the leak is a hoax. Nothing this grand and spectacular had been done, until it now it had. I think people underestimate the talent and patience of people.
It is a fact that a video presenting the appearance of his current job location was available by an associated bank was taken down. While this could be used in favor of the leak, I believe this is due wanting to remove negative attention they may have gotten from Super Smash Bros. fans.