You can’t. With the add-on no IP is specified, which makes it listen to 0.0.0.0. Within a Docker environment that usually is enough. I don’t know if the underlying Docker daemon of hassio has IPv6 enabled (has to be enabled). If so, it may make sense to actually hardcode the listening address to :: so it will bind to everything.
Nothing. This has to be changed in the add-on itself.
It may be possible to directly attach to the add-on-container and manually change this line from 0.0.0.0 to ::. But I’m no hassio expert, so you’ll have to finde someone (probably on discord) who can help you how to do this modification. If it works like you need it, please report back so we have a valid reason to make this the default.