We just got Carrigan. That would have been Donald's spot. Unless they realm wanna pull Langford AND Brockers on third down, I don't see it.
Denard or Dix here makes way more sense.
It's just more options for them the way I see it, and I'm not disagreeing with you regarding whether or not Donald is the best fit, as I think Hageman would be a better fit and long term solution, but of course I'm no scout
Carrigan is just an obvious depth upgrade over Conrath and Cudjo at any down, and honestly even the smaller Donald would make sense if you have Hayes and Sims filling in on tackle from time to time. If you get rid of that depth issue, then Hayes and Sims stick with backing up Long and Quinn making the entire defensive line significantly better, which is frankly a scary thought to me.
And regarding whether or not they would be better off drafting a CB, FS or DT this goes back to their obvious philosophy of trying to draft the best player available at a position of need. Of course if they're smart they have plans and back up plans all day long.
Regarding the secondary, you're preaching to the choir, but in Snead and Fisher I trust to know where they want to draft help at safety or tackle.