It's a bit of a "chicken and egg" situation to an extent, it seems to me. Are the O-line's struggles attributable in part to Manning's accuracy problems because those problems allow opposing defenses to key on the run, as you have suggested? Or are Manning's accuracy issues mainly attributable to a line that has allowed a very high rate of pressure so far this season? I tend to think it's some of both, which dovetails with my overarching point that the cause of the offensive malaise is multi-faceted. There have been times when Manning's inaccuracy has been caused at least in part by being under pressure. But there have been several other occasions when the blocking gives him time and he inexplicably throws bad, inaccurate passes.
And, of course, there are other factors relating to the offensive struggles, including some headscratching playcalls/strategy on occasion by Sark, the general lack of a consistent run game (which I don't attribute completely to Arch's struggles -- go watch some of the lackluster run blocking if you have the Kentucky and Florida games saved), and occasional drops of catchable balls by receivers.
When I started commenting on this issue, the two main points I wanted to make were: (1) the cause of the offensive struggles is multi-faceted, i.e., it isn't just Manning and it isn't just the O-line, etc.; and (2) Manning's performance last season indicates that he's at least got it in him to be a proficient college QB. My opinion on those two points hasn't changed, but I continue to be mystified as to why his accuracy has eroded even in situations when he's got a clean pocket and time to work through progressions.