Hmmm. I forgot I ever posted that.
Once upon a time, automatically generating a cover in Calibre from an FFF epub with title page gave a vaguely readable image of the title page as cover image. Then Calibre changed to render the title page as if it were on a larger display and "title page as cover image" wasn't legible anymore. Those settings were my attempt to make the title page readable again. I eventually quit trying.
Seeing the margins change in Edit is good. Not seeing it in Calibre View(reader), suggests either you didn't save it and re-open View, or the cover image thing, or some CSS weirdness. Edit and View
don't render exactly the same.
When editing a book, you'll see a OEBPS/cover.xhtml file or some other variant of 'cover.xhtml' if there's a cover image. In View(reader), it's apparent as the title page is still after the cover.
The fact words aren't being cut off suggests that issue is either Kobo, or the overall width somehow being set wider than the Kobo screen.
Can you post an example of your epub (SFW and ideally a small one) either here or somewhere else and link to it?