defaultuser0: it's a temporary account Windows uses during first-time setup, and it never has a password. If you land on its login screen, setup has crashed or looped, so there's nothing to "unlock". You need to get setup to complete properly, and that's why the Shift+F10 attempts and reinstalls kept dropping you back there.
For a consumer Surface Laptop with Snapdragon, Microsoft says the machine normally ships with Windows 11 Home. The commercial version ships with Windows 11 Pro.
Since this is a brand-new consumer Surface Laptop with Snapdragon, and you have already tried reinstalling Windows, disconnected Wi-Fi, and using Shift+F10, I would stop trying to work around the defaultuser0 password.
I'd treat this as an OOBE/setup problem rather than a problem with your Microsoft account.
If this is genuinely a brand-new consumer Surface, I would also check whether the device was previously enrolled in an organization/Autopilot environment. A reset/reinstallation does not necessarily remove an organization's registration from Microsoft's cloud. Microsoft specifically describes organizational enrollment/join during OOBE.
Therefore, before doing another reinstall, I would ask Microsoft to check the device's enrollment/Autopilot status using the Surface serial number.
Since the laptop is brand new, Surface Support is also free to contact and can often fix this quickly.