Service Request Field Validation

I'm wondering if someone could offer up a good approach to the following service request field validation requirements.  (note: SR has been renamed to 'Case').

CASE FIELD VALIDATION requirements:

a.       If case [IQLinkedAnswerFlag] = ‘N’, prevent Case [Status] from being = ‘Closed’.

b.      If Case [Status] field is equal to 'Closed', the [Call Type] case field is required (i.e., Not Null). 

c.       If Case [Status] field is equal to 'Closed', the [Product Area] case field is required (i.e., Not Null).

d.     If Case [Status] field is equal to 'Closed', the [Team] case field is required (i.e., Not Null).

e.*