You can change the z order by using the pageItem.move command with another item as a 'relative object' parameter and one of the ElementPlacement enumerations such as ElementPlacement.PLACEBEFORE;
However, the order-guaranteed selection is probably not possible unless you had some way to add to the selection and tell items apart. The problem is, Illustrator totally disregards the selection order and always sticks to the stacking order!
I think to tap into the low-level operations within Illustrator which could distinguish among a selection based on user-selected order, it would take C++ knowledge to create a plugin.