Just to clarify a couple of points…
Your carts are safe forever on your tribe land. They only become abandoned if the tribe does. Carts left on public land are deemed “unsafe” in the object header, but they aren’t abandoned or claimable for about 2 RL weeks. I think it’s specifically 135 game days, but I’d have to look at the old patch notes for sure.

Bins are never safe off of tribe land on the ground. As soon as you drop it, it is accessible to anyone regardless of what permissions and/or locks you have set.

Food items, whether meat, fish, plants, cooked or uncooked, all decay. As already pointed out, the rate of decay can be slowed by commissaries and/or specialty baskets/pouches. All will decay and break in time though. Note: raw food that is even down to its breaking point will become pristine again when cooked and give you a new decay timer.

No highlighted borders (would be nice), but you can tell exactly by coordinates. Tribes of 4 toons or less have a tribal plot of 50m x 50m square with the totem always at the center of the square. It’s pretty easy to fence in on a border by constructing a ghost fence or post and then moving it outward until you get a message that you can’t build off of tribe land. There is your border.