There have been lawsuits for GPL violations, so it seems their first point is wrong.
Most of those I've seen use the revenue used by the defendant as the example of damages, though, right? When I went through Microsoft copyright bootcamp this was like premise one. So folks like Technic and FTB who may collect money directly for their work (e.g., offsetting hosting costs and promotion is something I know both have asked for) are probably more subject to this.
Their second point is that mods do not have copyright protection - there is consensus in the modding community that they do, but it has not been legally tested.
You don't need to do anything special to have copyright of your code. I think Plus+'s point is that civil suits are non-starters without damages which sort of end-runs around it? Their idea that mods are purely derivative work is something that absolutely should be declared by Mojang though, because only a court case could draw that line definitively in absence of licensing.
Because they're actually slightly wrong about you not holding copyright on derivative work. As the music industry can attest to: the situation is more complicated than that.
Their fair use defense would apply to Minecraft itself, not to mods - Mojang cannot dictate how Minecraft is enjoyed by people who legally own a copy of Minecraft. Modders cannot dictate how their mods are enjoyed by people who legally downloaded them. Mojang cannot prevent people using a modpack with Minecraft, if that argument is correct. It doesn't say anything about acquiring mods.
It's tricky because you're right; redistribution is clearly in the auspices of copyright law. But Mojang's license explicitly forbids you from making a profit off the mod directly. So you're in this grey place where in order to claim damages you'd have to admit you were breaking Mojang's terms and making money off the mod. I do not have the legal chops to know for sure how that shakes out.
The other problem you've got is that it's both a pro and con to get included in a modpack. If that modpack blows up the way–say, Big Dig has? Well then that's great publicity for you and it's likely you can use secondary channels to get more revenue from your work. On the other hand, there is always the chance that that people will improperly claim your work or poorly handle the situation. I don't envy you in that dilemma.
But let me make it clear: you are definitely on the list of people that I would donate to if given the opportunity. I just suspect that's complicated without raising Mojang's somewhat justifiable ire. I wish TechnicLauncher had a button and a way for you to say, "I'll accept donations to the cause of my awesomeness" or "I'll accept donations to the following charity" and then we could probably offset some of your/other modmaker's adf.ly revenue, which I cannot imagine is very big.