Firstly note: As others mentioned you would be foolish to rely upon anyone's advice in a forum 100% without consulting legal counsel.
That being said here is some thoughts based upon my knowledge of your questions:
According to US Copyright laws any art is copyright immediately upon the author's publishing it. No notice is needed.
Also filing for the copyright is NOT required. Only if you wish to sue someone, then you must file for the copyright to litigate.
International Copyright law closely resembles US law but there are minor differences here and there. In some countries you must post notice is a somewhat common difference.
Secondly: In US Case law it has been upheld that a company can not sue you for "look and feel" of your game resembling theirs and win.
The most famous case that set precedence is where the copyright holders of Tetris sued a company for making a Tetris clone. They lost because its not enough to say "their game looks like ours".
This does not mean that they couldn't sue you and put you out of business with legal costs before you won your lawsuit though. This is in fact a common tactic of large companies to try to prevent clones from entering the market.
Now specific points to help guide you:
1: Copy & paste is a strict violation.
2: Fair use portion of copyright includes being able to use a likeness of the artwork in order to parody it. (case law cites use of photographs of copyrighted works to parody them being upheld)
3: "reference" of other's work is not a violation. However in the case of for instance you use Photoshop with a upper and lower layer and you trace the copyrighted work 100% as a 'reference' clearly your work will still be a violation if you make an exact copy or something that is clearly their character.
For instance say you wanted basically to make an Iron Man game. You cant make Iron Man ; but you could make a guy in a metal suit.
Important things to be sure it was not Iron Man would be to make his face distinctly different. The pattern and method of joining armor different. And probably the way he flies or moves different (instead of jets out of his hands and feet maybe a rocket pack or metal wings).
In this example they couldn't win a suit and say "Well he looks kind of like iron man because hes a man wearing a metal suit!" Because as stated above - existing case law proves that "look and feel" suits will not win in our court system. You have to actually copy their work to loose the suit.
Finally TradeMark law is MUCH looser on the side of the trademark owner. UNLESS the trademark owner does not rigorously defend their trademark.
If you notice an item is trademarked, honestly I would just stay the hell away from it
Hope that helps!