Where do zwave entity ID remapping values get stored?

When you add a new zwave device and change the entityID in the UI, where do those values get stored?

.storage/core.entity_registry

Is it safe for me to open and modify that when the system is off?

It’s not meant to be edited, but you certainly can, just be careful with the formatting. HA assumes it’s not edited so any syntax issues will cause trouble.