Add-ins in the new Outlook for Windows

KyleSwenson thank you for your queries.
Regarding #1 - It is not possible to restrict installed add-ins from being added to the message surface. Admins could, however, restrict access to add-ins by - 1) blocking the app store so that users cannot install any add-ins from store, or 2) making only some store add-ins available for install. Here are some relevant docs - Specify who can install and manage Outlook add-ins; Manage how users can install and use Office add-ins

Regarding #2, I would like to clarify that the entry point for add-ins in the email read mode, is available only in the reading pane action toolbar and not in the ribbon. This is what I mean by the reading pane action toolbar -

In the mail compose mode (when users are drafting a mail), the add-ins are available in the ribbon -

We are prioritizing availability of apps deployed by admins. Apps deployed by integrated apps or centralized deployment will be available in the ribbon (for mail compose) and in the reading pane action toolbar (for mail read) in the new Outlook for Windows and Outlook on the web. Here's the roadmap item for this feature: Microsoft 365 Roadmap | Microsoft 365