I consider a runoff as another primary with fewer choices on the ballot, so if you had previously voted in the Democratic primary you cannot subsequently vote in the Republican runoff. However, I am not a judge in Mississippi so you can't take my word for it. /s
Only voters who voted in the democrat primary are inelgible to vote in the Republican Primary runoff, any voter who did not cast a vote in either primary are elgible as are those who voted in the Republican primary..
There are two rules, one enforceable, the other not.
The runoff is considered a part of the original June 3rd primary, and the law clearly states that anyone who voted for the Dem on that day cannot then turn around and vote in the GOP primary on June 24th (It would be the equivalent of voting for two candidates at the same time, effectively.)
The other rule is the one you cited regarding intent, and the MS courts apparently consider that one unenforceable because it would be impractical to know a voter’s intention as to how he would vote in a future race. That provision is even being used to try to skewer McDaniel voters, including McDaniel himself, who now claim that they might not support Cochran in November if he’s the candidate. But, as I said, it’s unenforceable, and not enforced, although I suppose someone could use it to create some political mischief in the future.