@Peter890
It is not true that they can't do it. There seems to be a huge disparity between what one CS rep says and another.
I had a 4 star review changed to a 5 star one because the guest made a mistake. The rep agreed, after writing to the guest and confirming with her (was already there in the message thread, but she wanted to double check) that she did indeed intend to leave 5 stars overall.
Okay, this was a while ago (last Summer I think) so perhaps Airbnb have changed their policy on this, but if I was you, I would fight this. If you can't get a rep to help you out, try Airbnb's social media, especially Twitter, which I have heard can be more effective, although I've not tried it myself.
RE whether stars default to 0, I don't think so. It was my belief that if a guest writes a review, they must also leave an overall star rating. I could be wrong, but in nearly 150 reviews, I have never received one with the overall star rating missing or defaulting to 0.
The individual star categories are another matter. The guest does not need to fill in all or any of them. They can pick and choose. Misleadingly, when a guest does not fill in a sub category rating, it shows you in your Progress tab under the review that the rating is 5 stars. It is not, unless they left 5.
If you keep an eye on the overall percentages next to the star ratings, you will see if the guest rated that category because it will have either gone up or down. If there is no change in the percentage, they didn't fill it in, even if under the review it says they gave you 5 in every category. I hope that makes sense!