^ While Marvel can't say the word "mutant," whether they can use them is completely independent of whether they are mutants. What they can't use is X-Men characters, not mutants.
. . .
Tell that to Joss Whedon
They have certain leeway, but most certainly can't use the term "mutant," but they have plenty of ways around that.
So far, at least as far as they are making the point in AOS, they are classifying super powered individuals, or at least have pointed out the distinction, as either being imbued with abilities, vs innately having them.
It's interesting, as the Inhumans are popping up, as they are pointing out this distinction, but they (the Inhumans) kind of bridge both.
They are having something done to them to bring out their abilities, but they are technically bringing out what was already in them.
I wonder if that's intentional, or not.
Any who, we know they aren't going to just call any mutant character they want to use Inhuman just to get around it, but we don't know for sure what they will do.
We do know that the twins in AOU will have been altered via experimentation, but I wouldn't be surprised if Marvel has planned it all out leaving a back door open, should they ever get the rights back, or work out another deal like they did with Sony. Considering how long Marvel had been making plans for Spider-man in the MCU, there's no way they haven't had similar discussions internally around the X-Men, mutants, and the FF.
Of course, I doubt Fox will be looking to reboot the X-Universe any time soon, especially with the success of DOFP, and Apocolypse, and Deadpool coming up. Really the only thing in the X-verse that looks potentially disappointing is the 3rd Wolverine film, and only because of their track record.
Considering Mutants' abilities are initially dormant, they could very likely claim, latter, that the experiments on the twins only awakened their abilities.
Given their age, they will likely also have to have an explanation as to why they were still dormant well past adolescence though.
Long story short, I won't be surprised if we hear the word "mutant" in the MCU in the semi-near future.