Another rhetorical dynamic is the GOP puppet masters trying to get Trump off the stage. That is harder to do if Trump has pledged to not go third party, regardless of whether it's the pledge or the law that prevents him from going third party.
I'm unsure of that. It may be that if you actually win the state's primary then you can't be a sore loser, while if you lose the primary then you can be.
i.e. If he wins the Texas primary and later is denied the nomination at the convention perhaps he can be on the ballot in Texas as a 3rd party candidate... I'm not certain and if anyone knows for sure I'd like to hear from them.
The law may state that if you LOSE a Texas state party primary then you cannot be on the ballot under another party's banner... it may not address the question of what is allowed if you actually WIN the primary.
I’ll pledge not to eventually support the nominee. Jeb Bush will not get my support. Neither will Christy and a host of others.