Is there a way to show Office 365 group calendars in the play calendar app? I'm trying to get a calendar overview on my android device which has to include my google calendar (easy), my personal office 365 calendars (done) and the calendars of the exchange groups I am a member off (no chance).

Is there a way (or an app) that will provide a widget for the launcher that shows all the events from all the calendars?
Ok, for now the answer seems to be "No". I had a chat with the support from Microsoft and found some forum post on their sites. The Outlook App cannot show group calendar entries. To make those visible in your personal calendar (resp. create an entry) you have to accept the meeting invite which does not make any sense e.g. for a team holiday calendar.

The new Groups App from Microsoft in turn does not provide a comprehensive calendar view for all the groups you're a member off. Instead you have to check each group in turn. Also the app does not come with a widget.

Stands to say that there is a feature request to provide a groups calendar overview for about a year...