Kobo books are hidden in file explorer lists and don't appear in the Calibre library when the device is connected to a PC. Is there any way of (1) making them visible and (2) removing Kobo's DRM so that the books can be read on other readers?
Yes, it can be done. But it's not exactly simple. You start by installing the Adobe Digital Editions for PC program on your computer and downloading the "Adobe DRM Epub" to your computer, which should open it in ADE for PC. Then comes the tricky part: install the Inept Epub DeDRM plugin for Calibre.
I have used Ultimate eBook converter directly removed DRM from Kobo books synced via Kobo desktop and Kobo eReader, no need to install ADE.