Short answer: As described Eden could not exist naturally, you need a consult with Magrathean engineers to see if they could build it for you.
Magnetic levitation of millions of islands cannot occur naturally. The conditions necessary for magnetic levitation are very strict. You need carefully balanced magnetic fields of opposite polarity to support the floating object. A small imbalance and the floating object will tend to flip over and then the magnetic repulsion becomes magnetic attraction and you quickly plummet.
As your moon crumbles, the pieces descending would almost always naturally orient themselves such that they would crash into Eden.
As I type, I see that HDE 226868 has already answered the impossible field strength required aspect. I would also add that unless you manage a monopole somehow, the magnetic field cannot be in the same direction all over the planet, you could in theory have islands hovering over the north pole and others hovering over the south pole, but really nothing in between. Such hovering would not be stable as they would all tend to crash together directly over the magnetic poles.
You could get more stable magnetic support by making the floating islands superconducting magnets due to flux pinning, though unless your planet is extremely cold (where oxygen would be solid) they would not be superconducting naturally.
Lighter than air islands are the only "realistic" way to have floating islands. Perhaps you could have a large scale biological source of aerogels that are somewhat stronger than ones we can currently make and are filled with a lighter gas, perhaps methane (hydrogen and helium are very difficult to contain over time due to diffusion). Aeorgels are not very strong as construction materials though. You definitely need a stronger platform than any aerogel we have. If you add natural clumping of the floaters, you might have respectably large stable floating platforms, though clumping behavior does not result in a strong bond. An aerogel of graphene or similar fullerene structure might be a good place to start. You also have the problem of your islands catching on fire due to lightning strike, etc. but hey stuff happens.
For high buoyancy, a dense atmosphere is desirable, Oxygen toxicity limits the amount of oxygen, Nitrogen narcosis limits the amount of nitrogen and the narcosis problem is more general, essentially all of the heavy inert gases have the same problem, so there are definite limits of habitable atmospheric density.
Overall, it seems to me that Eden is not aptly named.
The tendency to flip over is much more pronounced that you might imagine. You pretty much have to cheat in one way or another to achieve magnetic levitation. According to Earnshaw's Theorem, static levitation is impossible for the most common types of simple magnetic materials. I apologize for being far too generous when I simply described it as a tendency to flip.
Superconducting magnets are not subject to Earnshaw's because of the way the flux lines penetrate the floating magnetic - causing flux locking. Rotating floating objects also bypass the assumptions of Earnshaw's. You can also construct a composite of a number of different magnetic sources on the ground and the floating object that don't match Earnshaw's model.
However, when you are talking about Eden as described, Earnshaw's is a pretty good model of what must occur. Since the model is not 100% accurate in this physical case, I described it in weaker language assuming that an edge case might float. It would almost certainly flip over due to perturbations from storms, etc. even when carefully balanced initially. So, floating for any length of time would be extremely rare at best.
Without going into the physics, you can levitate some things (such as a frog) using very strong fields that is diamagnetic (again, Earnshaw's does not apply), but as you can see it is not stable
Of course, the other problems such as the impossible field strength, etc. would still prevent Eden from existing.