I think that I got it right.
I got rid of weird spacing on book description using the metadata plugboard.
Code:
program: re(field('comments'), '\s+', ' ');
I also entered the plugboard in order to get multiple authors right on Kobo. It works flawlessly.
Code:
program: raw_list('authors', ', ')
I configured the Kobo Touch Extended Plugin to create collections based on tags that I previously and manually entered in the metadata. I also configured the plugin to update the metadata on the book details pages and to add subtitles, in my case page count, year of publishing and kind of book (novel, essay, manga...).
What do you think? My only concern now is that I can't figure out the difference between add subtitles with the plugin or with a metadata plugboard. It shouldn't matter, right?
Thank you guys. Kobo is a great device but thank to all of you it's even better now.