Sitecore Workflow User warning:
Today I did thorough testing on Workflow email. If you use the Workflow ContentHistory to get the submitting user like this as found on the web:
It's a partial solution. It only works the first time approval is requested. Once the item is rejected, the last item in the history will be the approver (and not the submitter!). Using Statistics.UpdatedBy is more accurate since it will always be the user who edits the item.
As to get the approver - this is not available as last history item nor the Statistics.UpdatedBy - just use the approver distribution email in the to field.