Just messing around with some numbers in my head...
Waller's base is more motivated, so he get's 70% of his August 6 vote in the runoff.
Reeves' base is more marginal and he relied more on name recognition on August 6, so let's say he gets 55% of his August 6 turnoff.
40% of Foster's August 6 voters turn-out again for the runoff, and let's be generous and assume they break 60-40 for Waller.
Results:
Tate Reeves: 111,268 votes (51.9%)
Bill Waller: 103,240 votes (48.1%)
This is probably Waller's best case scenario.
I think you're vastly underestimating turnout. In your scenario, turnout drops about 45%. I think it decreases by about 10-12%.
Waller's path, which is obviously a longshot, is 1) 100% of his voters show back up, 2) 80% of Reeves show back up, 3) 100% of Foster voters show back up and break 2-1 for Waller. That gets Waller to 50.1% and a win under 1,000 voters.
Of course, this gives you absurd results like Waller nearly winning DeSoto and Tate after doing very poorly there on 8/6. But it'll take something like that happening for him to win. That or massively overperforming there in Jackson (75% in Hinds/Madison, 60% in Rankin).