Unable to open iCloud drive files

The most likely cause of your problem was a lack of access to the internet or possibly that you might not have allowed enough time if they were videos

It seems you have chosen to store your iCloud Drive files only in the cloud, as such when you need to open them the Mac will need to download them first, in the event, they are large videos this may take a while. However, if you have no internet connection you won't be able to download them at all.

The 2 options available to you to ensure this doesn't happen again (assuming the availability of internet may not be within your control, is to either turn off storing them in the cloud only or ensure they are already downloaded before you start your meeting.

Is it good idea to include " Desktop and Documents" folder on iCloud ? ( By ticking the option in iCloud settings ?)

It's not necessarily good or bad, it's a personal choice. My personal choice is not to use it but rather use some folders of my own to keep documents I want to share with other devices and computers.

Will Desktop and Documents files remain available when we have no Internet ?

That depends whether you choose to keep them only in the cloud or not