It's basically Nikki Haley vs Trump at this point. New Hampshire may be Haley's best bet when it comes to beating Trump in any one state at this point. Polls had shown Trump being anywhere between 10-20 points ahead, although that was before desantis dropped out. If she can win there it'll help her in other states down the line as people will see her as a more viable primary candidate.

If Haley can't build up some momentum and win a few states, it may not matter if Trump is disqualified from being on a few state ballots. He wouldn't need them to win the primary and most of those states are almost certainly going to Biden either way, so it wouldn't have much impact on the general.

Donald Trump and Nicki Haley. And Trump is not harmed in the least bit as long as he's only left off blue state ballots. That has zero effect on his ability to win electoral votes.

Yes because Colorado was never going to go to Trump anyway. Maine splits their electoral votes but 1 or 2 electoral votes are very very unlikely to make a difference in the election. And that probably won't matter anyway because SCOTUS will almost certainly rule to keep him on the ballot.

And is Trump still considered a serious contender when there are entire states who definitely won’t have him on the final ballot?

He's the only serious contender. sigh

Those states, and counting, are forced to keep him on the ballot by the Supreme Court.