In the Inhumans vs. Mutants conflict, do the Inhumans actually have any valid reason for what they’re doing (or rather, not doing), or is it just Marvel trying to drive the X-Men’s popularity down?

I don’t think it’s the latter, since they’re relaunching the x-line with X-Men Gold, X-Men Blue, Cable, Generation X, Jean Grey, Iceman, Weapon X, etc. The closest to that you could get is to say that it might have been the plan, back when Perlmutter had more influence, and then things changed when Disney restructured and Marvel and Fox made their TV deal. I think we’re far off from a Fantastic Four deal, let alone an X-Men/Avengers crossover deal, but recent signs have been quite encouraging. 

As for the Inhumans, yeah, that’s a bit of a story problem. The writers created this zero-sum situation whereby the expansion of Inhumanity went hand-in-hand with the decline of mutantdom and the survival of mutandom meant the destruction of the sacred Terrigen Mists. Which is a nice dilemma, but you then have to have the characters do something about that instead of just wringing their hands about it, and that didn’t happen in Death of X.