If SD trades with SF for the #1 pick and selects Edwards or Williams, that will put added pressure on Minnesota to move up to guarantee geting the other WR. If that happens then neither WR will be there at 7 or 8, but one of the RB's and one of the QB's will be. I would still trade down again from 7 or 8 because I don't want one of the top RB's. I just don't think you need to spend such a high pick on a RB.
Personally, it wouldn't even take both of Minny's 1st's for me to move down. I would be willing to move down to #7 for Minny's 2nd this year and a 2nd next year (ideally we would get more). Then move down again another 4-5 spots and pick up another 2nd. Then take Alex Barron in the 1st and with our 3 2nd round picks (one from Surtain) I would take Matt Jones, Adrian McPherson and the best available RB.