If the GOPe try any shenanigans to cheat Trump out of a fairly earned nomination, he will be within his rights to consider the “no independent run” agreement to be null and void.
Even if Trump did not run as 3rd party, so many people would be disillusioned by the ham-handed GOP as to stay home rather than vote for whoever the kingmakers in the smoke-filled back room appoint, thus throwing the election to the Communists (aka “Democrats”).
If he wins nearly all the primaries but with less than 50% and is kept out of consideration at the Convention I would hope he goes independent. I am less and less pro Trump as he talks more and more like someone who aspires to be the ultimate Washington Insider but if he wins as big in the primaries as it appears likely then no other candidate is going to win a heavily Soros/Democrat chicanerized election and he would be the only choice to beat the Democrat.