The best way to sum this all up is that US copyright (and, really, IP in general) law is an intentionally tangled, loophole ridden, corrupt, Mess. Whatever its original purpose, in its current form it exists pretty much to give Disney, and others of that ilk, a way to lock down it's monopoly on, and functionally keep charging rent on the product of, access to work done decades ago. Probably by people who aren't even part of the company any more and almost certainly don't see any of the money.
There are really only three protections against lawsuits:
One: obscurity: if they don't know it's happening, they can't sue anyone over it.
Two: Being part of a large enough group that the costs (of all types) of doing anything about it out weigh the benefits. FF.net basically uses a combination of this and taking sane and reasonable measures in response to legitimate complaints.
Three: Be wealthy, canny, and stubborn enough that if they Actually take you to court, the lawsuit won't sink you and you've got decent odds of winning and setting precedent they really don't want.
If you're not managing at least one of those, then your only protection is hoping the legal/pr department, or author, or whatever is sane and reasonable on the matter. Generally speaking, individual authors are either very good about it, or absolutely crazy (though there are a couple who are actually reasonable and polite but still won't allow fanfics, it's almost always at least in part based on either bad past experience or bad advise by their publisher's legal/pr department.*), while big publishers of movies, games, cartoons, and books are usually fine with written fanfics on various internet communities or (back in the day) in fan magazines, but get downright Erratic about anything else (up to and including legitimate reviews which don't manage to be sufficiently "identical to our advertising, but free"). Let's not talk about the music industry.
*Seriously, even lawyers who Specialize in copyright law don't understand large chunks of it and acknowledge a lot of the rest as being awful.
Ultimately, monetizing fan-fiction is a risk. If you have the resources and mentality to take such a risk and deal with the consequences, go for it. You might even manage to set precedent that helps people out one day. Otherwise, don't. Because it's a grey area at best and there are plenty of companies involved who can and will use it to extort money out of you if they notice and think it worth the effort, either to get the money or to set an example so as to discourage others.
Edit: I'd say "consult a lawyer", but as I said earlier, even those who Specialise in this stuff haven't got a clue half the time. Any given lawyer will probably tell you either not to (because they're looking out for your interests, and having it all go pear shaped is not in your best interests), go for it (because they see a way to benefit themselves) or that they have no idea and it's up to you (because, well, they have no idea and are at least honest about it).