You are correct in that:
- Denied entry due to lack of ties to home country would not affect a K-1.
- A person can visit the US via a B2 while a K-1 (for example) is pending. There is a Mega-thread on that very subject here. In addition, many, many people visit while K-1s and spousal visas are pending. However, every visit is at the discretion of CBP if they think a person is just trying to bypass the long waits associated with the legal immigration process.
- Denied entry would not be a guaranteed ban. However, your GF is already on CBP's radar. The most troubling aspect is "The agent was aggressive from the start and insinuated that my girlfriend was working illegally". There are documented cases, here on VJ, in which that exact issue has resulted in expedited removal and an automatic 5 year ban. CBP needs only suspicion. That would affect any re-entry attempt in the future for 5 years.
- There is no clear guidance as to what determines abuse of a B2 or what circumstances dictate denial of entry. Those are purely subjective on the part of CBP officers.
Good luck on rest of the journey. This is a great place for info and sharing. Most of us are not licensed to give legal advice, but we sometimes spot bad advice given by those who are.