[Katsunami]

After the third e-mail (starting with: "It is not possible to follow the steps you have provided"), the Kobo helpdesk finally realised that it's impossible for me to bookmark books that are not in my library.

They have added them manually, and the issue is resolved.

Summary:
The Kobo helpdesk was polite, and responded promptly.
The people there were able to resolve the issue, in the end.
If the first rep had actually read the first e-mail, in which I said that the books were missing in the library, requesting that they add them, the issue could have been resolved with their first reply.

Ah well. It did end well, at least.