I don't think it's a problem that stone tablets protect each other. One of them has to go away first anyway.
It's possible I didn't get what you mean though

Now I realize though, with the new wording, tablet protects pillars from quicksand as well.. and that's not good.
also, there is no difference between the copper and stone versions... not that I deem it very important.
maybe if it read "if at any time stone tablet can be targeted by the opponent, it must be targeted" it would solve problems.
it would be useless PA'd and it wouldn't prevent from trident and quicksand.