The problem is hughes you don't think about the situation at the time and what Mayhew can control. He can't control injuries (Delmas,Best,Broyles). And the extent of Titus Young's problems was a complete unknown. Being a young immature guy is much different (some good college players were the same thing) than serious mental problems (that may have been triggered with a concussion or just Young not taking the proper medication).
There are tons of guys who had one injury in college and didn't have injuries in the NFL. And 2010 Best was the right call (Lions desperately needed playmakers), 2010 draft there was a ton of good trades for good players (CW,Rob Sims, Houston 2010-2012,Shaun Hill).
At the time, Schwartz was a good proper hire. He was an up and coming smart DC. He wasn't a head coach yet so he didn't get to show his full d-bag side. He had to be the good guy under Fisher. Or Fisher just didn't care. And Schwartz got his extension AFTER a playoff year BEFORE 2012. Bad timing if anything but with his contract running out, makes sense not to make him a lame duck guy. Linehan is a good OC for a lot of things. Making a veteran QB better is something he lacks though. There was lots of offensive production for Linehan when not hindered with injuries. And Gunther was a Schartz guy all the way. The coach chooses his coaching staff with suggestions from the GM. Schwartz/Gunther were buddies but the experience of Gunther was needed with Schwartz being a new HC. EVERYONE was excited by that coaching staff in 2009-2011.
And you didn't read my listing by year correctly with a big picture in mind. The Lions are set at 21 of 22 important positions (includes nickel CB and third WR but not FB) is the take from that. They only need a 3rd WR (and it would be nice if someone could take over for Calvin if he got hurt).