What word in English does the following: becomes plural by adding an S in the middle?

What cheese is made backwards?

As a stone inside a tree, I'll help your words outlive thee. But if you push me as I stand, the more I move the less I am.

What is greater than god, more evil than the devil. The poor have it, while the Rich need it. If you eat it you die!

I am stronger than an individual mare, and I am the same from the front or the rear. What am I?

What's a box with seven holes?

I clear the name of guilty men and mixed with common items I am deadly. What am I?