Yes, user_pass in the users table.
There is an external_id stored in Discourse, which can be associated with the Wordpress user ID.
Table: single_sign_on_records, columns: user_id and external_user_id.
Almost. Would be additional records in a table (user_custom_fields) associated with the users (user_custom_fields.user_id = user.id) and user_custom_fields.name='import_pass'.
Yep.