To get when a user was created: https://docs.q2developer.com/api_references/q2_sdk/db/db_objects/user.html#q2_sdk.hq.db.user.UserRow.CreateDate

The short_name is not based on the user or customer at all. That is a reference to the data. The purpose of Customer Data DbObject is to allow for DB storage on a per customer basis. Combines the Q2_CustomerData and Q2_CustomerDataElements tables. The idea is you give it a shortname for a reference for a particular kind of data you want to store and then that customer id and then you can retrieve that data later, or modify it or whatever. This is nearly a carbon copy of User Data, and we have an explicit guide on user_data here: https://docs.q2developer.com/tutorials/online_banking/database.html

Multiple user can have the same customer id. The relationship in the db is a one customer to many users. https://docs.q2developer.com/api_references/q2_sdk/db/db_objects/user.html#q2_sdk.hq.db.user.User.get_users_under_customer

get_multi refers to getting all the customer data for a particular customer. The relationship in the db is one customer to many customer data rows.