I picked up a copy of the New Jerusalem Bible last week–donated from my brother’s in-laws since I don’t have many Catholic commentaries, and they’re the sort of people who collect commentaries without really paying attention, so everyone was surprised to find out it was Catholic. 
Anyway, as I often do when picking up a new Bible, I looked up whether they said anything about Mark 9:49-50. Amusingly, all that commentary for the page started on the next page, unusual for the format of that Bible, with those verses leading out. Talk about divorcing the explanation from the source!–the normal reader would look down to see if there was any commentary and not seeing it move on. (There was however a mark at the verse indicating commentary.)
The editors’ brief comment was that the much preferred interpretation of those verses involves purgatory of the Catholic faithful–which I expected–but they also suggested the interpretation that God thus ‘preserves’ the unfaithful in unrighteousness forever.
I’ve seen some weird ways at trying to account for those verses, but I sure hadn’t heard of that one before!
How does this connect with us having salt in ourselves and being at peace with one another? The editors opined that Mark had ported that saying over to this scene merely because it had something to do with salt. 
How preserving the wicked in unrighteousness would be “the best of things”, the editors did not attempt to explain. They did however completely omit the explanatory “For” connecting the saying to the unquenchable fires of Gehenna, despite the Greek version of that term, a post-positive {gar}, being blatantly present. (Despite some historical precedence, RCC doctrine doesn’t recognize purgatory to be the fires of Gehenna. So of course they either have to explain away the {gar} or ignore it in translation or interpretation.)