David, please do not assume that Moodle test developers will not be able to understand that
Then "element" "selector" should exist
and
Then "element" "selector" exists
are two phrases for the same step.
Gherkin is supposed to look like natural language (a subset of) and as such it already has some ambiguity. Given, When, Then, And, But are interchangeable, but only most of time.
I'm inclined to think that only people who've taken one too many philosophy courses (I for one ) would be bothered by the difference between the potentiality in the first and actuality in the second, or even notice it. Others would probably just use one or the other.
Note that I'm not suggesting to deprecate and replace the "should" terminology. Merely to extend the vocabulary. And most importantly, the extension does not require touching the php. Just adding a few more @Given declarations as aliases.