You may want to check into the
ACSM Input plugin for calibre. This will still require to to have a valid Adobe ID for encrypted books.
And the Kobo desktop does not store files with a kepub extension. Instead you will find the files stored with what looks like a GUID/UUID as the file name with no extension in a directory called kepub. A sample file name would be 2eb5e2ef-8c1f-4dd8-a683-bce4c5746906 which does little for a human being.
As for ADE for Mac? When I look at Adobe's website, I still see a link for ADE 4.5.11 for Macintosh on the
Adobe Digital Editions 4.5.11 Installers page. The Mac version download link works for me and installed on a Mac OS virtual machine.