how do I have a workitem attribute(eg. defect Type) set as mandatory?

You have to control it from 2 sides.
1. Manage this project Area --> Pre-conditions and FollowUp Actions --> Save Workitem --> Required Attributes for type and State (This will make it mandatory field).

2. In Attribute Customization You have to write a dojo Script condition like
If owner == "ABC"   return false
else return true

and add this dojo script in Required attributes for condition (Pre-Condition). and select that attribute that needs to be bypassed.

Note : If you need further clarification , Please let me know.