If i were to rank all of the position groups as it realtes to importance of winning a chmapionship it would be:
1) Offensive Line
2) QB
3) Defensive Line
4) Defensive Backs
5) Wide Recievers
The offensive line is so important, your offense can literally do anything with a dominanat offensive line. And, as the OP states, we have had an abysmal to below average offensive line over the past 15 years. We won't win anything until we get the horses upfront again.
I definitely agree with your top three, and I like where you ranked defensive backs if that includes linebackers.
It's no coincidence that the last time Nebraska won a conference title was also the last time they had a pretty good offensive line. The one constant with all championship winning teams is their line play. Ohio State last year, Florida State the year before. Look at just about any good team out there and they've got great talent on the lines. Naturally, they have playmakers elsewhere, but it's crucial on the line.
Since Nebraska won it's last conference title, they've been anywhere from below average to above average, but that's about it. They've missed on recruits or a lot of recruits just didn't pan out for whatever reason. Until this gets rectified, we're not winning a conference title.