I'm really not a fan of getting a backup QB, and by drafting Stafford in most likely round 12, you are missing out on RB depth such as Michael Bush and Willis McGahee.
As B3N said, having two WRs of the same team is not a very good idea. Even though Braylon Edwards is on your bench, I would still recommend trading away Manningham for someone like Mark Ingram (not a big fan), Fred Jackson, or Marshawn Lynch. These guys could fill in as your flex. I would much rather have a RB flex, plus this gives you 5 RB and 5 WR.
Now that you have Stafford, hold onto him. Hope he does well. If he does, trade him. The upside on this guy is amazing, I actually believe if he performs well you could get someone like Felix Jones, and that would be awesome for your team.
Just putting in my two cents.
Draft September 2nd! Last Year 2nd Place 9-4-1 in regular season