What you are trying to do isn't going to be easy using checkboxes. The way checkboxes are coded WeBWorK submits a string of which answers the students selected (e.g. ABD), which is compared against the correct answer string, so either it matches and the student gets full credit or it doesn't and they get no credit. Short of making major changes to the answer checker you're stuck with this.
When I want to do what you're asking I create a separate PopUp (i.e. drop-down menu) question for each option. Then the default answer grader assigns the same value to each part, and students receive partial credit.
In your case for each expression there would be a drop down menu where the options are something like "is equivalent" and "is not equivalent". If you need help creating the PopUp menus beyond the linked help please ask.
WeBWorK Main Forum
Checkboxes with partial marks
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