I also believe it will never happen. In addition to the reason you give I will add (4) Cruz has coopted Trump’s positions on several things (i.e. immigration) and then claimed that he has always had this position. Also remember early in the campaign Cruz coopted Make America Great Again and had to be asked by the Trump campaign to not use that phrase.
If I were Trump this would strike me as a betrayal or extreme dishonesty and I don’t think he has a lot of tolerance for that.
It’s not that Cruz has taken these positions, that is a reasonable and logical thing to do. It is the manner in which he has taken them.
I would find it difficult to sleep at night if I betrayed my country by voting for the liberal trump.