How to handle Graph API authentication fallback in Outlook 2019 for an Office Add-in?

Question

I'm developing an Outlook Add-in that uses Microsoft Graph API for data access. The Add-in currently implements Nested App Authentication (NAA) using MSAL v2, which works fine in modern Outlook environments. However, I’ve run into an issue with Outlook 2019, where NAA appears to be unsupported.

To provide a fallback for Outlook 2019, I tried using the Office Dialog API along with a standard OAuth2 flow. Unfortunately, the dialog fails to open in Outlook 2019 with the following error:

"ADD-IN ERROR. A problem occurred while trying to reach this add-in."

At this point, I’m unsure if this approach is viable for Outlook 2019 or if there are known limitations that prevent it from working properly.

I'd really appreciate some guidance on the following:

What are the recommended authentication strategies for Outlook Add-ins targeting Outlook 2019, where NAA is not supported?

Is Office Dialog API + OAuth2 a feasible fallback for Outlook 2019, or is this flow known to be incompatible?

Is it a common or recommended practice to fall back to EWS for legacy Outlook versions (like Outlook 2019) instead of using Microsoft Graph?

Thanks in advance for any insights or shared experiences!