My guess with Rust is that they prefer him on RW unless they can move a better player than Rust to RW, at that point they're fine with him on LW. Sprong isn't better than Rust, so Rust isn't going to be moved to LW for Sprong like he would be for Guentzel or Brassard.
They should be willing to move Rust to LW for Sprong, but I doubt they'll be willing to. Sprong playing on the 4th line isn't even necessarily a problem, the problem is Sprong getting 3 minutes a night on the 4th line. They need to roll their lineup like they did against Vegas.
The more I try to make a lineup that I like, the more I dislike that the Penguins committed for as long as they did to Hornqvist. Or rather that they committed to Hornqvist like they did and didn't move any RWers. You have 3 solely RWers in Sprong, Kessel and Hornqvist, I don't like that. This isn't anything new though, I've been saying since June that they need to move a RW for a LW.