I think Heather's the only one of those four with no real right to sue. Sure, it was another technicality she didn't know about that took her out, but still . . .
Courtney was in a really good spot to at least make the merge (though since most of the other campers didn't like her that much and only tolerated her, she'd have been one of the first to go home by that point, anyway), and it got shot to pot because of some vengeful little weasel's stupid grudge against her boyfriend. And no, production knew what'd happened to her. They knew good and well what happened at the time of her elimination, but did nothing. So she had every right to sue.
Owen got physically injured on the show, something he'd usually have no right to sue over. However, as has been said, it was by a staff member, not by a third party. So he'd have every right to sue. Chris knew that, too, which is why he bribed him with sharing the Gaffers' reward that week.
Leshawna? I think she'd have somewhat of a right to sue, given that none of the eliminated contestants even meant to vote her off. (Damn Katie and Sadie for being dumb enough to get the ball rolling, though.) She'd have the right to sue mainly because Chris knew good and well that they weren't meaning to vote her off, yet ran with it, anyway. Probably to keep Heather on as the villain, which is, for Leshawna, even worse.
Overall, I think I'd place their rights to sue, from most right to least, like this:
1. Owen
2. Courtney
3. Leshawna
But one and two, as far as I'm concerned, can actually be pretty interchangeable, since both Owen and Courtney would have very good cases.