Hiya,
For 1, you are correct if the box is unchecked the users will not be archived. Even if that box is checked and the users are not enabled they will not be archived.
Here is a good resource for provisioning groups:
https://www-secure.symantec.com/connect/articles/enterprise-vault-granular-provisioning-best-practice
As for 2, Enabling a mailbox for Archiving will not affect their PST's. PST's have to be intentionally targeted.
Check out these articles on pst migration:
https://www-secure.symantec.com/connect/articles/white-paper-v80-pst-migration-part-1-solution-overview
https://www-secure.symantec.com/connect/articles/white-paper-v80-pst-migration-part-2-tools-depth
https://www-secure.symantec.com/connect/articles/white-paper-v80-pst-migration-part-3-planning-tech-tips-and-best-practice
Regards,
Tony