Given that the countryside is enclosed and the former rural poor (and their descendants) can't return there to live off the land, how do you foresee they will find enough land to live off - and the power to take over and use - in the cities?

The answer to this question will vary from place to place.  City parks, schools, vacant lots, unused parking lots; there are, I'm guessing, plenty of places in most cities where some sort of intensive permaculture can be practiced.

And don't you think, finally, that in the event of a breakdown in capitalism, the current system of industrial extraction practised in the countryside under the name of farming would also encounter partial collapse, and that the people of the slums would be tempted to go out and take the land they needed to live off?

Perhaps, but I have no idea as to the quality of the remaining land...

"Imagine all the people/ Sharing all the world" -- John Lennon