Can I save the Diagnostic and come back, or take it again?

Yes to both. It can be saved and returned to at any time, and nothing is final until it is finished.

1. Open the Diagnostic from the sidebar.
2. If a saved draft exists, choose Resume assessment. It shows how far it got and when it was last saved. Start again is next to it for beginning afresh.
3. Once finished, choose Review your results to read them, or Retake assessment to answer again. Previous results are kept while doing so.

A tip: answer for how things actually are today, not how they are intended to be.