I pretty much agree with this thought process, so that's my only 'knock' on JJ McCarthy (who I have not seen play) is that getting him at #16 basically is a bad draft move. Grab the best guy in his position, preferably a position of need, and then get cute later.
My only caveat here is that if Schneider and Macdonald really like McCarthy and don't believe he'll be available later, we are exactly the type of team where this guy works. Because he is only 21, he really needs to be able to sit behind a solid starter. Having just declared, yes, Geno is going to be here for sure next year, there is no pressure to make McCarthy start Week 1. We can be patient with him.
So, I wouldn't hate this choice, because I would trust that Schneider and Macdonald have a solid idea of what they're doing, but I'd lean towards what you said above as my preferred draft strategy.