I don't think it has to be DL or S but would be ok if it was. There are centers in the 4th round that will be upgrades over what we have so we could still get three starters out of this draft, which is what we were looking for.
As far as trading back into the second, it's easy...you just offer next year's first. NE is sitting there with the 16th pick of the second, after making two picks in the first and having two in the second, you know they would be a likely trading partner with their history of trying to get extra first rounders every year. Our 2013 first by itself should be enough to get their #16. Maybe throw in Jenkins if you have to.
Outside of that, there really isn't a combo that will get us back into the second, unless we use our 3rd and 4th and by the old chart that only gets us up to the top of the 3rd. Adding Jenkins or Jones might get us into the middle of the second at best.
I think there are lots of reasons to consider using next year's first. Seems with the free agency moves and the first draft move we are trying to make a push NOW. If we succeed, our draft pick will be in the back half of the first round. Secondly, we should have a better cap situation and can fill holes next year in free agency to make up for the loss of a first rounder. There are a number of guys that have already dropped that had first round grades in many opinions so the value is there.