If I repaint a scene from AC as a template it will be my work. I have full copyright to it. Except for the Characters in the image. But this only matters if I would make a comic with them for profit. There is many fan work out there, which use the pics from the 10th anniversary as template, if this what you imply would be true, then all of them would be illegal.
Yes. Often, fanart / fanfic / etc.
is a copyright violation, and the original copyright holder certainly could sue to prevent them being distributed. Usually they don't, of course, because it's not worth the time / effort / money to do so (and the internet being what it is, the usual game of whack-a-mole would ensure that they wouldn't actually stop it anyway).
Why do you think otherwise? They're usually very clearly
derivative works. Of course, if you're not selling them, the original copyright holder is less likely to
care.
The same would count for the countless Mona Lisa pictures you see in the commercial. If you have done the picture to 100% by yourself, then it is your work.
Well, the copyright on the Mona Lisa has expired now - copyright is for a limited period of time, in theory at least...
And this means if Square Enix would decided to use these new renders they have to ask Mayo Master if they could use them.
True, because he would own the
new creative elements that have gone into the renders. That doesn't preclude them also being derived works of Square's original graphics.