The one I purchased had a thickness of 3 5/8 "
The description does not state any pressure treatment and the prop 65 warning is for sawdust only. Also note that they do not contain the markings typical for pressure treated lumber (just the rough cut from the sawmill). They should be safe for use around a vegetable garden, but will rot over time if you do not prevent them taking on moisture from the bed (I'm currently in the midst of replacing a bed that has rotted out using these timbers.
Yes you could. Installing them into a solid footing would be proper to ensure no movement.
Yes, these are suitable to use for outdoor use. They are also able to be inserted into concrete.
Thank you for your question. Please contact your local Home Depot ProDesk for availability as this would be a special order item.
You will have to secure them together in some way. Like drilling through a few and install a rebar.