Does the architecture only support SIP communication or can it also support PRI? If so, how does that work since the breeze architecture diagram only shows communication between Session Manager and SIP entities.

Breeze can support PRI trunk calls, but it's less than optimal. Our "Deploying Avaya Breeze" document (https://www.devconnectprogram.com/site/global/products_resources/avaya_breeze/releases/3_5/docs/deploying/index.html) includes instructions to configure Communication Manager to route inbound / outbound ISDN trunk calls first out to Session Manager and Breeze. Just search for "ISDN".

The downside to this is likely obvious. This creates 2 different calls on CM where there used to only be one, and it also consumes 3 trunks (1 ISDN and 2 SIP trunks) where it used to consume only the one ISDN trunk.