How to set everything to silent except the alarm clock [7.1.1 Nougat]
You can try to achieve this by automation. I prefer MacroDroid being easy to learn and free

I don't have Nougat , so can't test it. I also recall  reading that dnd mode is behaviour is different compared to Marshmallow ( and don't remember how), so this may need further tweaking based on your revert



The way I understood your need and accordingly configured macro is


You are in Priority mode ( not alarm only )
Your calendar app sends a notification which needs to be seen by not heard ( select your  calendar app in Trigger, in the macro I chose default calendar app)