I don't believe any of those "possibilities" have sufficient bearing to account for what's happened this year. The line quite obviously played much better in pass pro last year than it is playing this year. You are making too many assumptions as to what Tannehill is or isn't already capable of doing, skill wise.
It's not a commentary on what he's
capable of doing in terms of movement in response to the rush. It's what he's able to do when his attention is allocated on such a big part of the game elsewhere, perhaps (his progression).
In other words, he's more than
capable of moving (i.e., he certainly has the necessary athleticism), but when his attention is focused almost exclusively on what's happening downfield, he isn't aware enough of
when and
how to move. Consequently he's a sitting duck when pressured.
When you call Tannehill "average", do you mean average when compared to other 2nd year starting QBs over the past 20 years or so? Or, average when compared to elite and established QBs of today (in essence: Brady, P. Manning, Brees)? If you mean the latter, why are you commenting at all, since he's exactly where we'd want him to be in his development, right?
To me, the only comparison that should matter at this point is with other 2nd year starting QBs over the past 20 years. Any other comparison is simply unfair, nor is it appropriate at this point.
I'm calling him average in comparison to the league as a whole. We need more data to say how he compares to second-year starters in the most recent passing era.
Be aware, however, that he was no better than average
last year, even in comparison to rookie starters in that era:
http://www.finheaven.com/showthread.php?330125-The-Talent-Surrounding-Ryan-Tannehill&highlight=
I suspect a comparison to second-year starters at QB would be no different.
That said, like I've said all along, if he finishes the year with a QB rating of 85 give or take a couple points, I'll consider it a success, and a likely sign that he can be franchise material long-term.