It's not simple. Say you have an expression that calculates a box's y offset based on where its neighbors are positioned. How do you move that box somewhere other than where the expression puts it?
I think you would need to control things with an external control of some sort. If only one box at a time can move, you could select that box with a slider and the expression on that box would say "I'm selected, I need to move 20 px". The neighbor's expressions would say, "My neighbor is selected, I need to move 10 px". Something like that. It depends a lot on how you are planning on animating things.
Would this be a case for the never-used Checkbox Control effect?
How do you write an if else with the checkbox control? (If it's turned on do this, if it's turned off do that)
Hmm were is the different between the Checkbox and the Slider?