Kobo recently started giving out epub3 files. Most of the old undownloadable books are downloadable as epub3 files now. You can download them with ADE2 and strip the DRM. No need for ADE 3 or 4.
I will check again now for you...
[insert 10 minute]
https://store.kobobooks.com/en-us/bo...g&PageNumber=1
This is available as epub3 for my region. I downloaded it with ADE2 and added to Calibre and stripped the DRM just now.
Works.