I don't think that the NBA can simply nullify the contract, it might take a long investigation though, as they needed to get all the facts straight before making this hard decision, but nullifying it might have a ripple effect on some contracts on the NBA specially those who are signing or teams that are giving big money to NBA players right now.
For now, it seems that the case is his words against the owners word. So let's see how the NBA is going to settle this issues. Regardless if James is not happy Sixers should allow him to get out of contract and that could be the best settlement for him.
I mean that would be unprecedented if the NBA got him out of that contract. I think it sets a bad precedent in that anytime a player gets unhappy they can try to wiggle out of theor contract. I would think the contract itself is the only underlying thing you can lean on. Talk is talk, people always go back on their word. If you want it documented insist that language gets drawn up in the contract.