No, you need to fix it manually. And don't forgot the shadow.

.plain

https://developer.apple.com/documentation/uikit/uicommand/adding_menus_and_shortcuts_to_the_menu_bar_and_user_interface

Was your mono app compiled with the macOS 11 SDK? If not, that's the answer. Big Sur will report 10.16 to older apps to avoid breaking version checks that were not written correctly.

Big Sur is 11.0. But if the app was compiled with a previous sdk, it will report "10.16" to avoid breaking some wrongly coded apps version checks.

iWork is not bundled with macOS. Apple will update it after Big Sur is out.

The link in the support page works for me, it opens the App Store and you can start the download from it.

The Genius Bar clearly needs better Geniuses… you can download the macOS Catalina installer from another Mac, make a usb drive installer, and install it. It's amusing that the Geniuses neither know of to restore a T2 and neither know how to install macOS.

No, there isn't any official deadline.

You can restore the T2: https ://mrmacintosh.com/how-to-restore-bridgeos-on-a-t2-mac-how-to-put-a-mac-into-dfu-mode/