Thanks a lot for all the info. Much appreciated. I managed to find the info on kobopatch but you were quicker than me.
Instructions are clear and configuration files are well documented/commented so I managed to do everything I wanted.
As far as Collections are concerned, I simply was asking if it's normal for files already on the device not to be editable as far as "collections/series" metadata are concerned.
I needed to copy existing files over, from PC to Libra, in order to update the Series tag. If this is normal behavior, no problem.
Now I just need to understand why the Bookerly font is turning italic text into bold/italic and I'll be content.