Very good question. The Broncos use Von Miller around the formation, but, for the most part, on passing downs, you can find him opposite Ware, going against right tackles.
I believe the best nickel lineup will be: Mack-Autry-Mario-Irvin. If Aldon proves to be back to his old self, then I might see a situation where Mack slides inside, or Irvin moves up to a nickel role (with potential to blitz). But Irvin doesn't seem to have the pass-rushing versatility of Aldon or Mack. I feel like Aldon and Irvin essentially are best doing the same thing, rushing from the right side.
It does seem like Mack moving inside, in some capacity, seems the easiest way to get all four on the field. Maybe a 3 man lineup, with Mario over the nose, and Mack hovering from a MLB spot, obviously coming, but from where? Really, in that scenario, just the concern from the interior of the O-line might be worth the wrinkle, even if Mack is a better pass rusher from the edge.
From a larger view, if I had to pick a free agent that won't be worth the money, I would go with Irvin. He has not been a special player in Seattle, and seems to be a one trick pass rusher. He's clearly a lovable guy, and a leader, and has bought in to the team, so he might be like a Tuck, not an impact player, but contributes in intangibles and steady play. We might need him more as an off the ball LB on early downs more than his use as a pass rusher on obvious passing down. Malcolm Smith has been a nice journeyman vet for us, but is a replaceable player. Heeney is an unknown, and even if he's good for us, he'll probably have a lot of growing pains this year.