Rounds could beat either Johnson or Herseth. If Rounds is smart, he would appoint an elder stateman Republican (why not John Thune's mentor, former U.S. Senator Jim Abdnor) to finish out the term, and then run for the seat himself in two years, turning the governorship over to his LG.
The LG would be running as an incumbent for governor, Rounds would take the high ground *and* be U.S. Senator for a very long time.
I like your plan, but I'm not sure if Rounds would beat Herseth. I agree he'd be favored against Johnson, but Johnson is a boring old man, not a beautiful young hotshot like Herseth. Also, Herseth's voting record is much smaller, and much less liberal, then Johnson's.
I'm not saying it's impossible, just that it's far from a sure thing. I'd say it's even money at best.