I wouldn't call Nelson a "dud". I mean, he obvious had some mental issues, but his talent was pretty clear. That the offense is stagnant no matter who is at QB, each with various abilities and varied talents (Gray, Nelson, Shortell, Leidner, Streveler) suggests either that the problem is greater at WR, or the problem is greater with the playcalling.
This is not a reply to you, more a global thought: Are we to really believe that Streveler is such a bad passer that he's only worth 7 pass attempts against a patsy opponent? For real now. You don't get to be a backup QB at any D-1 school - much less in a Power 5 conference - without some ability to play that position, command an offense, and throw a football. The players are better than the ultra-conservative playcalling suggests them to be. Those patsy schools we played in OoC pre-season were moving the ball pretty well, that with lower-recruited players, and against a much better defense. They didn't move the ball by being scared to let their lowly QBs throw the damn ball.
Finally, note that I think lack of high-end talent at the WR positions does play a significant role in this. However, it's a catch-22. If the offense shows that it never wants to pass the ball, then you're not going to be able to recruit premier QBs, and you're not going to be able to recruit premier WRs. And without those premier players, the team will still refuse to pass the ball, and the cycle will be virtually unbreakable.