Think about this, though. If the Catalyst is controlling the Reapers, then the synthetics he's actually referring to ARE the Reapers. It doesn't mean much if the Reapers aren't a unified hivemind with a singular goal; the Krogan aren't either, but their uncontrolled ravaging of the galaxy was a certain enough possibility that the Salarians slapped them with the Genophage, not once, but twice
It probably isn't so much that every individual reaper is dedicated to devouring organic life. It's just that MOST of them are, and the Catalyst is the only thing keeping their appetite for destruction partially in check. In which case, The Catalyst isn't as concerned with preserving organic life so much as keeping the Reapers from exhausting their food supply.
I doubt that the Leviathan of this file and the Leviathin of Dis are the same Reaper. The Leviathan of Dis was probably Sovereign, or simply another Reaper whose "real" name we don't know
Reapers don't have real names, they are merely content to use the names given to them by others (even the name "Reapers" is merely what the Protheans
called them). Harbinger, too, is only so named because he continually refers to himself and/or the collectors as "The Harbinger of your ascendence."
It's too much of a coincidence, IMO.
Actually, they do have names. In ME2 Legion says Sovereign (the name made up by Saren) referred to itself by name as Nazara. One assumes that is the name of the race that was used in it's creation. Harbinger is an odd one since, being an english word you'd think it was another made up label, but the reaper you kill on Rannoch says to Shepard "Harbinger speaks of you."
Perhaps it's real name directly translates as "Harbinger"?