Just face it, you won't get meaningful replies here unless you're lucky enough to find someone who's reading this and actually studied licensing laws.
My good friend is a lawyer who helped Westeros: Total War with their Cease & Desist letter. It pretty much boils down to this: You use a company's IP, it is within their right to tell you to stop and you pretty much have to do it. The thing is, even if you say no, they can shut down your website by telling your hosts, then the hosts would then have to face the legal challenge and they will most likely just turn you off, less of a loss. Your forum has advertisements on it? You are profiting from the IP by people visiting your thread in regards to that mod.
The only way you can win is by compromise or reaching an agreement with the company involved.
US case-law also has a lack of legal precedents when it comes to this field as well, because simply no one is stupid enough to pay thousands of dollars in legal and lawyer fees to challenge an IP holder over making a mod. There are a few very good lawyers who would be interested and take on such cases (such as my good friend) but as a realist, it isn't something you want to consider.
