I began to explore the terrain for a Word plugin for the company I'm working.
- Although there is a common format for the manifest file, Outlook and Word addins seems to be required to have it separate. I didn't see any addin in AppSource that's both Word and Excel addins.
- There is a tool from Microsoft called Script Lab that allows to run arbitrary scripts and testAPIs in Office.
- For authentication, it's better to open a browser window to your web app via Dialog API
- It's usually not possible to use iframes to get authentication tokens from the taskbar/sidebar itself. See this answer.
- It's possible to add elements to right click menu in Word for add-in commands.