'What does Johnson actually want?' is a very good question.
He wanted to be PM, more than anything, agreed?
We know he wrote all those Euromyth stories as a journalist just for shit-stirring basically, and we know he only made his mind up to back Leave rather than Remain at the last minute - I suspect as a completely cynical move to give himself the best shot at PM.
Then he lets TM attempt to build the cheese submarine, with predictable consequences.
He swoops in at the last minute, hero of the day...then what?
You don't become PM hoping to go down in history as the person who broke up the UK and condemned Little England to a future as an impoverished, irrelevant backwater, surely?
He must know what a disaster a chaotic Brexit would be, and he surely knows that even a controlled Brexit will screw the country over for a long time, except for his rich mates (I grant that might be an acceptable outcome for a Tory - but I'm not even sure his Tory convictions run that deep either)
But who better than good old floppy haired joker Boris to stand up in front of the TV cameras and say, 'Look guys, we had a really good go at this Brexit thing, but due to everyone else being crap, it just wasn't possible. Jolly disappointing, eh? Champers all round!'
Secret Remainer.
You heard it here first. 
(Oh, but don't tell anyone because the plan relies on him being seen as rabidly pro-Leave
)