Is it possible to get  form submission diff with Diff module
Problem is - diff compares entities and their revisions. But node_form_submit() happens before revision is created. If you will inspect node_save, you will see no hooks are called after actual save.

To get your code executed and be sure revision is there, it would be tempting to use Rules, but existing bug "After updating existing content" runs before content is saved. shows it has just the same issues as hooks. Maintainers say:


  We just rely on the hooks drupal provides us here.


So now you have 2 options:


Give up on using diff module and just borrow some of it's code (it's open source after all)
On save, set a hidden field "test with diff" to true, and on cron make actual testing. With */5 minutes CRON it will be almost as good, as instantaneous test.