The biggest problem is that the magnets will "quench", which is what happens when a superconducting electromagnet suddenly stops being superconducting.
There's a lot of energy stored in that magnet, and when it quenches the energy all turns to heat in a very short time. Any remaining helium will flash boil, turning into an explosive expansion of gas, and the thermal shock will seriously damage the machine
No, it would have been detected by various systems pretty much immediately. Those systems are military though, and probably wouldn't tell the general public about the movement of military satellites
It's also conceivable that it was detected in that orbit but not recognised, so it was treated as a mystery object