My fiancé is coming to America on a fiancé visa and we plan to get married within the ninety day limit of her visit. Can she then return to China to attend her son's wedding before receiving her green card? It has been suggested to us that she may get her passport stamped by the Chinese Embassy to allow this travel. Is this a possible alternative and acceptable/legal procedure?

... Can she then return to China to attend her son's wedding before receiving her green card? ...

Certainly. A green card is not a travel document such as a passport or visa.

But the real question is whether, once she returns to China, she can then come back to the US. I believe that, as your wife, she could get a new visa for that purpose, but I will defer to other members who may be more knowledgeable than I am regarding immigration law.

Also, if she will be taking your surname upon marriage, you should consider whether a notation to that effect on the passport &/or visa might be possible and advisable.

Well, she can return to China, because she is a Chinese citizen with a Chinese passport. But it will cause problems with her with respect to immigrating to the U.S. for two reasons:

* K-1 is a single-entry visa. That means once she enters on it, it's void. She will have to get another visa if she wants to come to the U.S. again. If she's married at that point, she will no longer be eligible for a K-1 visa. Then she can only immigrate through Consular Processing in China. If that is the case (she is only coming temporarily to the U.S. to marry and then leave), she might as well do it on a B-2 visa.
* Once she applies for Adjustment of Status (the process of getting a green card inside the U.S.), which is the normal thing to do for people who marry on K-1, if she leaves the U.S. while Adjustment of Status is pending, she will automatically abandon Adjustment of Status, unless she gets Advance Parole before she leaves. Adjustment of Status is a large amount of money and effort, so it would be a huge waste to abandon it. Advance Parole allows her to travel out of and into the U.S. without a visa, before she gets her green card. She can apply for Advance Parole for free at the same time as she applies for Adjustment of Status. It takes around 2.5-3 months to get after applying.

So basically, she should not leave the U.S. until she gets Advance Parole or her green card. So she needs to plan for about 3 months after applying for Adjustment of Status (which in turn must happen a little bit after the marriage, because you need enough time to get the certified marriage certificate) when she must stay in the U.S.