Yeh, the complicating thing for Trump is that DeSantis is not considered to be rino and he is genuinely liked by lots of republican. Whereas when he ran in 2016 that large field of clowns were made to be smacked around by Trump. If Trump loses in the polls/primary to a clown proving that the republicans are losers, then, yes, he will and should just walk away and tell them they are on their own. Where it gets dicey is when/if he acts like he cannot support any Republican candidate unless it is himself. I don't think he would go third party but I can see him playing that or hinting at scenarios that sound like that, as a way to keep his flag flying if he gets less traction in the polls than he expects.
Don't know. He entered the race so ungodly early there will be years of gyrations to deal with.