This is my understanding
QUESTION 1) . A login event is not generated by such users but notifications are being sent to them ; they being part of some group. Do such members consume Windchill License ?
Yes, they do. If you have an active user in WTUser table, it is a license.
QUESTION 2) How do you classify the audited user list as in 4 distinct roles, (Heavy user, Light user, View & Print user and External user) ? Windchill is licensed as per these roles.
I am not able to make a sense of some of the definitions, but you can Refer to https://support.ptc.com/support/customer_agreements/basis_files/jan-11/Licensing-Basis-en.pdf
QUESTION 3) How to best generate the compliance report and optimize license compliance ? What process do you follow to maintain accurate Windchill role count ?
Disable/Delete inactive/unused Windchill accounts. If you can't delete, put them in a group called inactive and deny the access from webserver.