I don't know how gladly Slash would be with GNR releasing an album he doesn't at least play on. If you mean the remainder of the tracks with Duff/Slash's re-recordings, then yes of course they'll happily go along with that. If Axl releases either of those versions that will be the last GNR record AND Slash/Duff will have written absolutely zero songs on it. The cost involved in a boxset is too much, and the interest level in chinese democracy era music isn't enough for any label to finance it. I also find it hard to believe Axl would be happy dumping these songs on some extras vinyl/cd for a boxset anniversary of Chinese Democracy.
As for doing no harm putting it out... maybe, but what if HardSkool and Absurd are the better tracks from the collection he has? are we so desperate for music that we want to get 14 other dud songs like those? personally, I'm past the "just release anything, I'll buy it" stage. Whatever they put out needs to be legitimately good, none of this willing yourself to like it BS, those songs really killed my enthusiasm for new GNR😄