Without looking up the formulas, your numbers look close enough.
Epoxy is designed to work in shear: Two surfaces sliding past each other, but restrained from motion by the bond between them.
Ignoring the steel band for a moment...
(50mm) * (50mm) = 2500 square mm = 3.88 sq. inches, contact area per magnet
If each magnet is trying to fly away with a force of 354 pounds, and the contact surface is 3.88 sq. inches, then the shear stress is 354/3.88 = 92 pounds per square inch
Most epoxies I know are good for thousands of psi. When wet, that drops to many hundreds of psi, but still safe for you.
With the steel band, no problem at all.
Just bear in mind that the typical failure mode of an axial-flux alternator isn't "just" the magnets flying off - it's overheating, which leads to stator warping, which leads to rubbing of the rotor on the stator. That's when the magnets are most likely to fly off.
Hey, you asked for an engineer's opinion!