As far as I could figure it out, if you first place a control out of the view, and then move it in using only the constraints, it does not become child of existing controls. Only when you move a control with the mouse does it become child. It is cumbersome but effective.
I do not understand why Parent has become readonly, though. It would be much easier to do Parent = Nil like in Desktop. But given the deafening silence from Xojo team about that, I guess we can forget about getting modifiable Parent. Maybe it is simply impossible in iOS.