Regardless of his attitude, they had to dump Mario in order to keep the young stars, as you state. That move made replacing him the #1 need, which as many have stated, coincided with the BPA--the stars aligned! This doesn't make him a star, but in Ryan's D he is a talented, high-effort player who fits better than a disgruntled Mario.
In Ryan's D, stopping the run is paramount, and pressure comes from everywhere. Shaq will set the edge as well as Mario, but most likely be more aggressive in tackling against the run. He will get his sacks, but I doubt he'll be a double-digit guy--Again, the sacks come from everywhere, so he will simply do his part. They still need an NT, and they also need KW's replacement, so they could go in that direction in R2. However, this pick can go in any direction, depending on how the round falls to them. A lot of DT talent left, but they could also use S and LB. If the Clemson CB is there, I wouldn't be surprised if they take him too. So many options...