Those 6 high stacks are the ones at the port. From the thread, there are plenty of smaller secondary lots outside the ports where empties are being stacked, because the ports won't accept them, and there's room. The zoning based stacking limit affects those secondary yards, but not the port yards. A real question is if the secondary yards have equipment to stack higher or if they can really only go two high, because they were limited by zoning anyway.

> It would also be interesting to know if it would be feasible to make the containers able to be disassembled and multi-packed into an empty container.

Not really; everything's welded together, and if you unweld and reweld, it's not going to be as strong. Plus that's a lot of labor. There are some collapsible containers, but those tend not to have sides or a top, which is not ideal for ocean shipping.