Very interesting project, congratulations!

I don't think I'd ever be brave enough to use it on my real WhatsApp account, as you say in "Account Safety". Losing access to it would be an unacceptable risk for many reasons (Brazil here, WhatsApp is 2FA channel, important for work comms, where I receive a lot of government messages, the only contact for many businesses, etc.).

Is there a contingency plan in case Meta doesn't go after users, but sends a C&D to get the project shutdown?

No contingency plan. I think I'd just try to talk them into adopting it or parts of it.

How likely is it that my WhatsApp account will be banned or shadow‑banned?

In 2 months and 65k+ downloads there have not been any reports of bans.

But I can't guarantee that! Use it at your own risk.

Seems really cool. I am skeptical about using external Whatsapp client due to hacks