These horn and bone ferules weird me out a little bit; I don't think I understand them, though they are often nicely made. I'll lay an alternate philosophy on you concerning filling cracks, which is the crack is from shrinking/swelling/warping due to moisture exchange, mostly with the air, and when those conditions change again, it might want to move back, and not be able to because of filler, and this might even cause a new crack, that, being one of two, would weaken the piece much more greatly, and endanger the ferule of being lost. This is something a very expert wood carver laid on me one time, and it's worked its way into my mind like one of those bugs that Khan sticks in peoples' ears.
Moisture exchange can be slowed with surface treatments; some might even amount to stoppage.....What of oiling material to swell it back to shape or something? This is a theory I've never much experimented with that might have some relevance; perhaps someone can tell us about it. Mine has a crack in its horn ferule, too, and it's also missing one, that I presume to have been bone, as it has a gap, the horn one, then a bone one, then actual (horn) handle surface currently. As a stop-gap to keep the ferule from rattling someone has put some little horn shims/wedges behind it.
Will you tell us about the suspension system on the sheath? Is that an iron staple?