Rather than worry about built-in water resistance, a milspec waterproof phone case would probably solve your first quesiton.
Can’t answer the second. Although wear and tear on the port seems to be an issue with old androids. Wonder how long the Librem 5 USB port will phyically last.
The water issue is about the durability and portability of the USB key itself.
Example 1: I attach it to my car keys → friend at party pushes me into pool.
Also, what happens if a USB key is destroyed or stolen, in terms of recuperating our ability to log into devices? Are we to keep some sort of encrypted backup in cases like these?
Encrypted or unencrypted, a backup sounds like a good idea. Presuming an unencrypted backup is in a secure storage facility and offline, like tape. Local and off-site storage is even better. With periodic rotation, perhaps once a week.