Did safties back up when we had Ginn... No. He wasnt considered a deep threat or any threat for that matter.
Marshall might not have speed to be considered a deep threat, but what he does know how to do is make YAC, which can be considered a deep threat in in of itself, terms of yardage.. same thing with Hartline, he averaged 16 YPC last year.
All your other concerns are completely valid.
That's a good question, I think it matter what corner was on Ginn, if the corner wasn't that great then yes safties on his side played a little deep, the NY 1st game helped with that as Ginn burned Revis..then as the year went on, some teams didn't respect his deep threat, and Ginn almost burn them again, i.e., pittsburg.. For the most part, they did respect his speed, but not his ability to get off the jam..