modelform: override clean method
For 1, Yes, if you want to make use of parent class's validators. See this explanation on the doc.
Warning
The ModelForm.clean() method sets a flag that makes the model validation step validate the uniqueness of model fields that are marked as unique, unique_together or unique_for_date|month|year.
If you would like to override the clean() method and maintain this validation, you must call the parent class’s clean() method.
For 2, yes, if data validates properly. Otherwise raise validation error.