If I understand it correctly, your suggestions wouldn’t have prevented it, which is evidence that this is not as trivially fixable as you believe it is.
Operate under the assumption all accounts will be taken over because centralized corporate auth systems are fundamentally vulnerable.
This is how you actually fix it:
1. Every commit must be signed by a maintainer key listed in the MAINTAINERS file or similar
2. Every review/merge must be signed by a -second- maintainer key
3. Every artifact must be build deterministically and be signed by multiple maintainers.
4. Have only one online npm publish key maintained in a deterministic and remotely attestable enclave that validates multiple valid maintainer signatures
5. Automatically sound the alarm if an NPM release is pushed any other way, and automatically revoke it.
Whats even more stupid is they actually started mandating 2FA for high risk packages, and FIDO2 supports being used to actually sign artifacts, but they instead simply use it for auth, and let releases stay unsigned. Even the developers they insisted hold cryptographic signing keys, they insist on only throw-away signatures for auth, but not using them for artifact signing to prevent impersonation. It is golf clap level stupid.
Consider them a CDN that wants to analyze your code for AI training for their employer and nothing more. Any security controls that might restrict the flow of publishing even a little bit will be rejected.