I am sorry to give you all this trouble.
I found that the first try was very convenient for my level of understanding (even then I did a mistake...) as it allows to process
one EPUB at a time. Furthermore, as we can use again and again exactly the same command, it's easy to use. We just have to replace an EPUB with the next one.
I'm afraid the "loop" thing is way beyond my capacity. I did not expect it to become so quickly so complex. Just writing long file names for the terminal (using antislashes and things like that) is a chore.
So, I will keep myself at the first level. Thank you for your help, explanations and brilliant solution that, for sure, I will use next time...
