[srichard]

All the kepubs I have were bought (none were converted from epubs). So all should be of the right format.
I buy them from Fnac, which is a bookseller associated with Kobo. From what I understood, they share their catalogue of ebooks and their database of customers; however, there has been a problem when synchronising their databases for my account and since then, when I connect to Kobo website (or when I use their desktop app), it does not see my latest purchases. However, when I connect to Fnac website, it sees my purchase and gives me a direct link on Kobo server, which allows me to download it "manually". I then have to transfer it to the device, it is where I use Calibre (I import the kepub to Calibre, then ask Calibre to send it to the device, sometimes after adding or modifying some metadata within Calibre; I did not install any plugin except yours).
The best is that I continue to hope that Fnac and Kobo will finally fix the problem they have acknowledge, so that I can use Kobo desktop app or direct Wi-Fi download again. No need to spend more time on my very specific problem, but if you want an example of my kepub, no problem (some are "purchased" but in fact are free). I will also try your suggestions with Calibre KoboTouchExtended driver.
Thanks again for your help and all your information. I understand better the limits of Calibre with kepubs.