I haven't read all, but Attack no. 6 requires access to unlocked phone. IMO, if that is the case, I wouldn't consider this as an attack, at least not as something that would stop me using the service
Edit: notably, this wouldn't be a problem if (1) Threema required a preconfigured password to encrypt the private key with, or (2) used an identity transfer scheme that was detectable on the target device. But since they're just copying a key, it's entirely undetectable.