ok quick update for everyone and also for the record since other people might have the same issue one day:
1. our plan of attack was adjusted and we planned to take our helper to Guanzhou (through Lowu) to see whether we could exit China with a re-entry permit and the European passport. If not, the helper would have gone back to HK with the baby (we organized a direct taxi from border to airport and back to make it simple)
2. Once we reached Lowu, the helper could not get an instant visa (bummer and we should have known but forgot to check). However since we were at the China border, we asked the China immigration folks there and they thought it should not be an issue to make the trip with the documents on hand.
3. Helper returned from Lowu and had a nice week off at home in HK
4. We did the whole trip as planned with the baby
- entering China at Lowu with a re-entry permit and the European passport
- exit China at Guangzhou with re-entry permit and the European passport
- enter and exit Cambodia with European passport
- re-entered China at Guangzhou with re-entry permit and the European passport
- exit China for HK with re-entry permit and the European passport
My conclusion: You do not need a HK passport to exit HK and enter China at any time if you bring some other passport with you.
If mother/father could not find their passport, we would have cancelled the holiday since the whole point of the trip was for the mom to make a final trip before giving birth to our second baby.
Last but not least for those who wondered: our baby is 17 months old