Your description of a flat spin is quite on the mark. The one place we might differ is on what would keep the aircraft in a sustained flat spin. I do not think that possible unless there is a failure of some kind in the tail section or the separation of engines from their mounts, or both. Otherwise, I believe, the "flat spin" would change to a normal spin or a spiral.
For clarification, Astramike's description of a flat spin is lifted word for word from;
DC-8 Mishap on 12 Dec 1996 N827AX -Stall Recovery in Mountainous Terrain
Flat spins start from normal spins as the spin develops. For a flat spin to develop from a normal spin you need a propellor blown elevator, (which we don't have on 447), and/or aft cg, (which we do). If the cg is aft enough the aircraft is perfectly content to stay in the flat spin all the way down. Inertia keeps the airplane in the spin. The tail section is not sized, nor configured, (rudder area below the HS), to recover A330's from flat spins. Engine departure will move the cg further aft, which is good for flat spins.
Note; Excuse my somewhat, what can be construed as, contrite responses, but I'm not one for words - try to say much in as few words as possible so as not to take up my, and my audience's, time. No intention of being condescending, respect the ones that deserve it, especially the pilots. Y'all are tops at what you do. Just trying to fill-in in places.