Hi Mrugesh,
There won't be any issues if you use a hashed customer ID or any customer ID, instead of hashed email address. But ACC will need a reconciliation key/file, so ACC should be able to understand and resolve hashed customer IDs to email addresses, vice versa.
1) why do we need offline traits and segments? As ACC will push segments into AAM anyways (Pls correct me if i'm missing here anything).
That's correct if you have the native integration in place then segment will get created by Adobe Campaign and that will reflect in AAM.
If you are not using ACC and AAM integration, and sending a file using ACC ETL process, then you need to create onboarded traits and segments in prior.
Even, with the native integration in place, if you create a segment in ACC, you will have to come back to AAM UI, edit that segment and select appropriate profile merge rule for that segment.
2) Once segments are already pushed into AAM - how do we segregate users who've responded to email campaign and those who've not?
You need to create that Audience or list within ACC. I am not very much across ACC, but it should have functionality to create audience of users who opened email, clicked and went to landing page and did not open. Check this doc on exporting audience to Audience Manager: https://docs.adobe.com/content/help/en/campaign-classic/using/integrating-with-adobe-experience-cloud/audience-sharing/importing-and-exporting-audiences.html
If in case ACC does not track who clicked and went to the landing page, then you you can create a custom pixel to track links and collect data in AAM. See this doc for more info:
https://docs.adobe.com/content/help/en/audience-manager/user-guide/implementation-integration-guides/media-data-integration/click-data-pixels.html
So, you can deploy custom AAM pixels on CTAs within email like: (subdomain).demdex.net/event?d_event=click&d_cid=(DataSourceID)%01(HashedID)%011&c_emailclick=yes&c_platform=email&d_rd=https%3A%2F%2Ftest.com%2Flandingpage.html
Thanks,
Varun Kalra