I'm staying away from this, simply because it will end up in court one way or another. How it ends up there, and where the court is, will influence how things go.
Looking up legal precedent, there is very little about paid modifications to games. I have to go waaay back to game genie for NES (which sided with game genie) about something like this. That was over 30 years ago...who knows what the courts will decide now.
Anyway, unless valve decides to shut all this down, it will head to court. If I was Valve I would hope it would occur in the USA, because in the EU I see what (at a glance) seems to be multiple gross violations of law.
This is a very, very messy system.
Addendium: The more I think about this, the deeper the rabbit hole goes. I'm considering examining this as a thought experiment, it really can become a legal quagmire. For example: now that is codified that a modder can release a product for profit of a game, what is the relationship of the modder relative to a company? A contractor with no specific contract? An intern? A volunteer? And how does Valve enter this? What is their relationship, legally? These relationships will have to be defined in court now that profit is in play.