Along with what others have said:
Use multiple magnets, in different orientations so that when you go to snap them back they self-align.
Magnets hold very well in one direction, but they do very little to prevent sliding. Adding some short pins or other small hook will prevent the prop from slipping down your back, but still allow for a quick release.
Do not allow the magnets to smack metal-on-metal. Rear-earth magnets are NOT solid metal objects like they look. They are nickel plated powder. If you break the nickel plating then the powder inside will come out and they will never work the same again. Adding a coating of rubber such as plastidip provides a small amount of cushion, and can even hold together a cracked magnet. They also help with the sliding friction. But be aware, if you put rubber on both sides, the rubber can fuse over time if you leave the magnets connected.
No matter how strong the magnets, they will not interfere with batteries, LED or other electronics. Unless your are planning on using 3.5" floppy disks, or a CRT monitor on your prop, you will be just fine.
Regarding releasing the magnets, look into electric door holders. They make models which use permanent magnets to hold, and electromagnets to release. However, most are just an electromagnet.