Ehh, I'm having trouble deciding. I feel like you really need another running back, mainly because I don't like any of yours. (although I think Bradshaw is probably fine, and my dislike of him is irrational, and induced by the existence of Brandon Jacobs who really doesn't steal much from him anymore). And I'm old school and still feel like having at least 5 running backs is a must.
I do think Jimmy Graham is an upgrade of Mercedes, and Jay Cutler would be useful for your bye week since Brady is a must start every week and Fitzpatrick has the same bye as Brady. He's playing Tampa too so its a decent start, it is in England though not sure if that effects it though.
I don't know if i trust Hightower enough though. Marshall is proven and will score points for you consistently. The first game gives some confidence that Henne can air it out.
If you trust your WR's enough, pull the trigger. I personally would just since Blount and Charles are inconsistent, it gives you more options.
(Could you offer Boldin instead of Marshall? I think I'd rather do that, Marshall has the potential for a much bigger season in my opinion.)