I like DG, but I think you hit several nails on the head.
First, DG underthrows a lot of guys who are wide open on the deep ball. We still pick up 40 yards, but it could have been a 65 yd TD if not underthrown. It seems like his deep balls are either underthrown giving the DB time to catch up and make a tackle or just a bit overthrown. Maybe in year 3 we will get more consistency. And I'm not trying to pile on DG, kid plays hard. I don't think he has the pocket presence or the ability to escape like MM had. I don't know if MM will get back to the level he had gotten to. He might be a half step slower since the injury (which would be completely understandable).
On an unrelated topic, crossing routes or underneath routes. Under Frost (not saying we need to bring him back) we challenged the D on every level of the field. It just doesn't seem like we ever hit receivers coming underneath. It's throw it deep or a WR screen on the perimeter. IMHO, it just doesn't seem like we make the D cover the entire field. I think we have one of the best WR corps in the nation (they drop a few balls, but they can all make plays). I guess what I'm trying to say is the play call determines the success of the play, not the player. Let Marlon or O'Keefe catch a pass underneath and then be the athlete they are. With our WR's, this offense should be more productive than it already is (Most yards per game in the country).
As far as the D, you can't hang Tulsa on the D. Offense didn't do much in that game, even when handed great opportunities. Memphis I do put on the defense. Cincy, I thought the D hung in there when the O didn't give them much support for large portions of the game. I reference Cincy because at that point we were playing with 4 more starters/major contributors kicked off the team. As a head coach you need to understand your team, not just the offense. You know you are shorthanded on D. Running 3 plays and then punting as fast as you can doesn't help the team. Maybe its what you want to do on offense, but you need to look at the entire game as a head coach. You are responsible for both sides of the ball. Maybe there is a time to go fast and a time to slow it down. Again, I'm not saying bring Frost back, but he (and I disagreed with a lot of his game time decisions, and even a couple of times I was right, joking) he understood there was a time to go fast and a time to slow it down because the D needed an extra minute or two to catch their breath.
But for except a possible bowl, the season is over. Now is a time to evaluate what we have coming back (coaches included). Hopefully the coaching staff can self evaluate and see where they can improve. Honestly, I feel the talent is capable of being in a NY6. But I believe the coaching and play calling needs to develop