

How much shorter depends on a lot, like moisture content, ground composition, stuff like that so I couldn’t say with any precision but I’ve seen PT wood rot to disintegration in less than a couple years. You could just kick the top of the piece and it just crumbled.
There’s options for you if you’re set on burying wood-type construction. Take a look at PVC or trex-style decking planks and see if you can design at least your uprights, corners and bottom boards in that material then if you want(for cost, probably) switch to wood when you’re a foot off the ground.
Regarding your stone, I understand the moss is a part of your wanted design and I think you can build it in a manner that you could use it. Like I said, just do your best to keep gaps to a minimum and pack as tightly as you can.
Whoever wrote the title was just trying to insinuate something extra vicious and violent in nature. It’s a common part of our fair and unbiased media.