If I unpublish a process with included forms - will that same process still work for processes outstanding in drafts, or waiting for completion?
No! Can't tell you the technical reasons but from experience releasing an updated form and unpublishing the old version while there were still active instances with it will cause problems. I usually change the Link name of the old form to something cryptic so no one can find and start a new process and then unpublish once all instances are complete.
Rui gives a little bit of detail here: https://answers.laserfiche.com/questions/125031/Using-current-form-link-for-newmodified-form
In general, if you unpublish a process, running instances of the process shouldn't be affected. Drafts will still appear in the user's inbox, but they won't be able to submit them to an unpublished process. If the message start event form changes, the drafts will need to be updated to the new form.