"Haley isn’t someone who will inspire an insurrection or who would deny the election," said a Democrat leading a campaign to get crossover votes for Haley.
This is not "messing with Trump". Anyone in any party should participate in primaries. They decide who everyone else will get to vote for, and in some regards they are more important than the general election. For example, if someone is in a gerrymandered district, primaries are the only elections that matter for district level races.
If one party literally has only a single candidate, and thus there is no choice, don't vote in that primary. It's pointless. Vote in the other primary, for whatever candidate you like best, even if you don't like the candidate. Then there's some hope that when the general election comes, you'll at least be OK with both candidates. In the general election, vote for the one you like best, and if the candidate you like loses that election, you made a difference in the primary. Republican, Democrat, any party or no party, it's the way to give your vote the most influence.
many states restrict primary participation to past voting record.
Connecticut for example: Section 9-431 of the General Statutes of Connecticut stipulates that only registered members of a political party are entitled to vote in that party's primary, though a party may choose to permit unaffiliated voters to participate in its primary.[10]
In those places one would need to register for the appropriate party an appropriate amount ahead of time. What the field will look like overall is taking shape well ahead of time, and in some places ballots will have already been set before party registration deadlines arrive.