Note: If the Open Shared Calendar or Calendar Permissions buttons are unavailable, you might need to change one of your settings. See You cannot open a shared calendar in Outlook 2016 for Mac for instructions.
![Calendar Calendar](https://support.content.office.net/en-us/media/57f0f0f3-30f0-49ba-85b3-4b1e074880ce.png)
What I would suggest is to ask the owner to move the calendar temporarily and if necessary make some permission changes on the parent calendar. It can always be moved back and the permission changes can be undone. Another option would be to create a dummy calendar with no sensitive information and share it as Microsoft suggests. If it works, you know you've found a viable solution. If, after presenting either offer, the owner of the calendar is still not willing to participate in a small test then I don't know what to suggest.
You are right that the problem does lie ultimately with Microsoft and possibly Apple to co-ordinate a fix.