[...] (the leather case can block RFID/radio waves and microwaves afaik).
Leather does not stop RFID cards etc. being read.
Leather does not block radio frequency.
You'd need some aluminum for that. You know.. the material
which also protects us all from the governmental mind control.
can magnetic fields destroy them!?
Short answer: Yes.
But this doesn't mean that you lose access to your coins.
As long as you have your mnemonic code (the 24 words), you will be able to recover your coins.
You should ALWAYS have the 24 words.. the device could always break or fail.
Having a backup of these words is the only way to guarantee access.