With a recent version of ubuntu-image, you can use the --disable-console-conf option to turn off console-conf which then means the prompt to create a user is disabled and the device will not allow new users to be created.
If you still want to seed your own user into the image, you can create a system-user assertion for this task. See https://core.docs.ubuntu.com/en/guides/manage-devices/
Not yet, but that is something we are exploring