I don't think anyone knowledgeable ever said he couldn't play short. I believe they were speculating that he'd probably end up around major league average, which is pretty damn good btw, and if another player came along who could play above average short, it could push McGonigle to 2nd or 3rd, where he'd be above average, and he'd hit enough in any case.
I think they see Barnes as a Sam and will probably run a lot of 5-2 this year. I think they'll look for a hybrid safety\linebacker later in the draft for use in the nickel.
What you are missing is that your conclusions are not based on facts, he doesn't have a lot of swing and miss, he has a relatively low k rate and he makes a lot of hard contact, he has superior strike zone judgement and he has been very young for his competition level. He also should end up as a plus defender.
It's not just to stop the run, though. In this era of mobile, escapable quarterbacks, they want ends that can control the lanes and gaps and crush and contain the pocket.
Where did anyone get the idea Hassanein won't be strong against the run? He's raw in every aspect, but stout enough, strong enough and motivated enough to develop.
They picked late in the draft and ended up with three starters in their rookie years. If they got a starter in Teslaa with a late 3rd and two picks after the 3rd round that is a steal. If any of the other picks end up helping at all, that's a bonus.
Last year wasn't so bad, Reed much better before he got hurt, Lopez, Ya-Sin, Maddox, Allen, Stewart all pretty solid, got some immediate help and at least 3 solid starters out of the draft, not all bad for tight budget and drafting late.