I've been thinking about this issue for awhile, and I can't help but wonder if it was Hackett's fault? Here's my case:
I remember Sammy Watkins saying during OTAs and camp that he had to work on his "steps." The passing offense was based on the WRs taking a number of "steps" consistent with the QBs drop (based on 3, 5 or 7 step drop-backs). I don't recall any discussion of this method in year one, and I am wondering if Hackett tried to come up with a fancy new passing system that was too mechanical? WRs and QBs having to focus on "counting" rather than playing. I can't imagine how long it would take to adjust to this kind of system? And, how does one adjust steps when being jammed at the LOS? The fact that all 3 QBs regressed tells me that it was a new system, which I think contributed to regression by the entire offense.
I would certainly give EJ a pass (so to speak) on last year, and am very interested to see how he does this year in Roman's system, which hopefully doesn't rely on steps...
Regarding the Oline and Marrone, I recall Marrone brushing off questions he got about moving people around during the preseason, trying to find the best 5 players rather than getting his 5 guys early on so they could develop consistency and timing during the preseason. He didn't think it was an issue. I think that WAS exactly the issue. I think the Oline regressed because of the lack of familiarity with who was next to you.
My 2 cents.