I would like to grant minimal SQL permissions to some users who will regularly need to run 'static' reports - that is, reports on email traffic and on OWA and ActiveSync activity.
You will have to grant at least the following permissions to your users in SQL Server:
This will allow your users to run all "static" reports - i.e. based on data imports. The corresponding report categories are: Inventory, Traffic Statistics, Mailbox Traffic, OWA Activity, ActiveSync Activity, Server Traffic, Internal Message Delivery.
Note: Reports of the Storage, Mailbox Content and Public Folder Content categories are "dynamic", which means that they are not based on imported data. However, you will definitely have to grant the PROMODAG account specific Exchange permissions to run these reports - that is: membership of the Organization Management role group - or alternatively of the Recipient Management + Public Folder Management groups. The reason for that is that the user account must have the permission to open other users' mailboxes, and to run PowerShell commands.