I wonder if it would be possible to include the option of essay questions when designing an assessment using the slide mode? I typically use the slide mode because of the graphics ability but occasionally wish to add an essay or writing portion to the assessment.
You can't add an essay in a slide. You can only add an essay in multple choice / short answer design mode.
However, you should change the way you think about the design area for slides and multiple choice. It is a place to create questions, but not always the place to assemble finalized tests. If you assign two tests (one of type "slides" another of type "multiple choice") to the same class, you can merge those two question sets into a single test. Select both tests and use the "Merge" button. You can rearrange the question order and even remove questions after you have merged them. Some teachers find it useful to create a class whose purpose is not to contain actual students, but simply to contain their important tests, which have been assembled from various parts (slides, essay, auto-generated math tests, etc).