I don't think you would have to.
First you show a pattern of illegal votes. If that pattern is pervasive enough, the runoff election needs to be redone.
2.Logic would tell you that, comparing the results from 6/3 and 6/23, what the trend was in each county. That would show which way the Dem vote was leaning. Not exact proof, but reasonable inference. That Thud got so many more votes in the runoff than in the primary, in counties which specifically showed a large increase in Dem participation in the runoff, is a huge clue.
3.Ask the Dems who committed the illegal voting who they voted for, in exchange for immunity from prosecution. That would prove interesting.
The illegal voting should be prosecuted anyway. Those bar codes next to the names of the cheaters are pretty conclusive and damming. And a few dozen prosecutions should help keep their elections honest for a couple decades? Maybe even a few county clerks who were dropping those double-dim-votes into the hoppers?
If I understand correctly, logic would also indicate that the votes were for Thad since the flyers, calls, etc.we’re urging votes for him and since the community was already saying that he “owed” them for their support. This was an admission of sorts.