Vertical scratch on the screen: is front cover sufficient?

It doesn't appear you got the answer you were looking for; as far as I can tell the front cover does not come with any glass or acrylic screen; it's nothing more than the shell around the screen.

As such, the only fix for the scratch you're seeing is to replace the entire screen. That basically means gutting the reader, putting in a new screen then rebuilding it with the parts you took off the original. The front glass on the screen is not considered replaceable; the separate layers are all bonded together, so you have to replace the whole thing at once. Here's the iFixit guide to that repair.

Kobo Clara BW (N365) Screen Replacement - iFixit Repair Guide

One note here; rather than answering, I almost skipped by your question because you had already marked the previous answer as accepted. When you do that, many of us volunteers will ignore it, assuming you've gotten whatever answer you were looking for, so you need to hold off accepting an answer until your question has actually been answered to your satisfaction.