How can I re-enter the country if my I-551 stamp expires?

You should be able to enter the U.S. on a valid re-entry permit. If your re-entry permit expires before you are able to travel back due to circumstances beyond your control, you will need to process for a travel document a the U.S. consulate. A re-entry permit will allow you to return prior to the expiration date on that document. You should try to do this if at all possible. If not possible, you are correct that you will need the consulate to issue a travel document.

If it appears the health issues will cause delayed entry beyond the date of expiration, it is advisable to seek an extension, perhaps within at least 60 to 90 days. Also, consider notifying the U.S. consulate in China about your condition prior to expiration date of re-entry permit.

Legally you are still a permanent resident and should be able to enter. However, there are issues such as being able to board the plane. The re-entry permit should suffice. Explore entering through Canada as you can explain the circumstances to an officer at the port of entry.

A re-entry permit will allow you to return prior to the expiration date on that document. You should try to do this if at all possible. If not possible, you are correct that you will need the consulate to issue a travel document.

As long as you have a U.S. re-entry permit and even though your I-551 stamp expires, you should be able to re-enter on your re-entry permit prior to Jan. 2, 2019, with an explanation as to your health problem. It would be good to have evidence from a doctor explaining same. If you try to come back after the expiration of your re-entry permit, in all likelihood you will be deemed to have abandoned your lawful permanent residency, you will ineligible to be admitted and would have to start the process of obtaining your lawful permanent residency all over again. You cannot apply for a U.S. re-entry permit when you are outside of the United States.

If you have a re-entry permit, you could travel with that. You may have to explain why you could not get another stamp in your passport, but because you have the I-829 receipt notice, you should be able to enter as long as it is before the re-entry permit has not expired. Also, you may be able to get a I-551 adit stamp at the embassy. They usually have an USCIS officer, with whom you could make an inquiry.