Clicking delete doesn't send any notification to the other side, where this account still shows as having accepted the invite (the other side is a different Office 365 organization).
That is interesting. I'm using the PWA on Linux - I can't share a screenshot but basically pressing the delete key shows two buttons: Cancel and Decline Event? - I don't get the Delete option like you do.
2. When you select a slot in the calendar screen and press delete, the only options you get are cancel and decline.
All this is applicable for the PWA. The native client works fine.