How can I use cross-chargeability to save my EB-5 case with a bankrupt project?

If you get married, and since I-526 is still pending, you can still use and apply for cross-chargeability.

If the jobs were created, you may be able to save your case and if you have a good faith marriage to a person born in Hong Kong, you can cross charge to their place of birth and will likely be able to obtain a green card within a few months of notifying the National Visa Center.

Cross-chargeability can be raised at the visa interview stage.

You probably could, but that will not help you at the I-829 stage.

Yes, you can. However, if the project fails and if it has not created the required jobs, even though you file your I-485 or DS-260 and get a conditional green card, you may still not get through the I-829.

Yes, it is possible.

Cross-chargeability will help with the priority date. But it will not make a person eligible for a visa for which one is not otherwise eligible. So, if you could not be granted adjustment on your own, you cannot be approved married either.