"What, exactly, would be the basis for a Khadr libel suit? "

Well, it would be the same basis as for EVERY libel suit, e.g., that these people are disseminating false and damaging statements about him. So, for example, calling him a convicted terrorist, pretending he wasn't a child soldier, accusing him of murdering Speer, those are all potentially actionable. 
Like, just as if I said those things about you. If they're damaging and false, they're actionable. 
"One might quibble over the fairness of the American justice system "
Quibble? QUIBBLE? 
Like habeas corpus is a detail? Like torture is a minor affair? Like indefinite detention without trial is a minor matter? Like trumping up fake crimes retroactively is not a fundamental breach of the rule of law? Like our refusal to repatriate him after every single western nation did is not a source of shame? 

These things are quibbles to you? 
"If Khadr is really worried"
Khadr is not the commenter. Try to stay relevant.