I am with you on adding an OG but I don't think it's going to be a Turner, Thuney or Scherff. I just don't see them investing there that heavily given their constraints and think they will likely look for a veteran OG that is released elsewhere (comp. pick formula) that they could sign in the $5-6M range.
My guess on Havenstein is that might not play itself out until the Draft itself. They may wait and see who they can get at C or LG in the 2nd or 3rd rounds and
then decide whether to trade Havenstein for a day 3 pick to a team that has missed out on adding an OT. Or perhaps they even stumble into a RT that drops to them and decide they can play Evans and Edwards at the guard spots.
Lots of moving parts. I've said several times I want a scenario where Corbett ends up as the 6th man vs the starting LG. They need quality depth and I'd feel good about Corbett, Noteboom and Allen as the 6th-8th OL.