I think Grimm is depressed. It's just not played out obviously. Stiff upper lip and all. He's just using humour and stoicism to hide his frustration at being the thing now. In the last issue Ben Grimm laughed for the first time since the accident, that certainly gives a show of his depression such as it is. and just because he laughs once, that doesn't mean he's not depressed at all. I think it's just nicely subtle.
But onto namor, I personally wouldn't like to see a great atlantian empire under the sea that no-one has found before, of course it's possible, even in our own world, but if that were the case, what do atlantians look like? They can have him as a mutant, but perhaps that's a little bit of a cop out as he would have had to have had humans in his family at some point in the past. and to have an undetected empire, it would need to be pretty far underwater. So, we can look at atlantians having a common ancestor with humans, but, we'd have to go a rather long way back down the evolutionary ladder to find something that could feasibly work, and this would take him far away from looking human. And I don't think he can be too far from human because of the sue and namor thing. But, if there is no empire, then it need not be so deep in the water, and then the common ancestor thing could be acceptable, however, how would a species survive if it was such a small community without developing genetic symmetry.
So maybe, a mutant he is. I suppose the only thing i can say with confidence is that he won't have little wings on his feet
(damn i hope i can say that with confidence)
Additionally, I would like to see that he does look more fish like, and not just a tough blue dude.