You know, the thought had occurred to me that perhaps there's something else going on here. There are two other potential hypotheses that I can think of which could cover all the behavior we've seen thus far, plus the shenanigans with the actor's name.
First, what if they're just screwing with us? No, really. On the face of it, asking us to believe that a completely different species--with multiple, redundant organs--can somehow pass as a human is...well, it stretches plausibility. Yes, it's been done, in TOS. But we're a much more sophisticated audience now. Asking us to believe that seems to be a bridge too far. Sure, they can still do it, but we'll all have to admit to ourselves it's a bit implausible, even taking into account all the unplausible elements that are already taken for granted about Treknology (like transporting, for example). So, maybe the producers don't think we're suckers to fall for that trope and, instead, want us to think Ash is Voq, just to play with our heads. After all, it's too obvious. So if it's too obvious...what if it's a head-fake?
Second, what if it's something worse than an implausibly surgically altered alien? What if Ash is a Manchurian Candidate? Not the original, Frank Sinatra version, which used "brainwashing." No, I'm talking about the remake, which used medical nanotechnology to operationalize their control over the patsy, Sgt. Shaw. That would account for the ease with which Ash seems to get human idiom and non-verbal communication. Voq, being a Klingon fundamentalist, would likely as not have no idea how a human thinks. But if Ash is just a puppet--with Voq behind the wheel, as 'twere--then we solve a lot of problems.
Imagine the reveal, late in the season. Everyone is expecting Ash to either dissolve into Voq or be exposed through some kind of medical scan but, instead, Voq shows up, stage left, and reveals some box that he uses to control Ash (and see through his, control his movements, etc.). It saves Ash for use in Season 2 rather than have the producers come up with an implausible "That wasn't Ash but here's the real Ash...who acts like the old Ash who was really Voq" type situation.
Just a thought. Also explains the shenanigans with the name. The actor couldn't stand the makeup, they didn't want to recast, so they came up with a way to make do with what they had, with only one more use of the makeup needed to complete the character arc, at the end.