Found this thread a little late and not sure if this will help, but....
After 2 hours of trial and error I managed to remove the rouge calendars from Lumia 930 phone.
Logged into Microsoft Outlook Exchange.
Accessed Folder view
Permanently deleted calendars from trash (for some reason the calendars did not show in the trash while in Mail view)
Next on ...
You are still running 1511 version it seems. A lot of bugs has been fixed on Anniversary update i.e)version 1607.
Do the following:
Update Windows 10 Mobile to version 1607.
Install any pending updates for Office on Store.
If things didn't work after that, try uninstalling and re-installing the office apps.
If nothing works out perform a hard reset.
Office on Windows Phone will be automatically connected to the OneDrive that belongs to your primary Microsft Account (the same one you use for store downloads).
To change where a document should be saved simply create a new document -> then open the app bar at the bottom -> select "save to..." and choose phone or OneDrive. If you choose to store the file ...
As far as I can tell you can't delete files from the Office Hub. But you are right, if you can secure the OneDrive app with a pin this should also be possible for the Office Hub. You should post this as a suggestion on http://windowsphone.uservoice.com/
Environment: Nokia Lumia 521 running WP8.1 DP with latest updates.
Confirmed: Excel on the phone is watered down. I've got a spreadsheet saved into OneDrive that has Conditional formatting and list inputs, and neither work on the phone. I'm happy to be able to have Excel on my phone, but I'm glad I don't need to rely on that spreadsheet day-to-day.