Immigration benefit may be a result of marriage, but it cannot be the sole purpose of the marriage as in "in order to obtain an immigration benefit...."
"in order to obtain" means that was the purpose of the marriage.....which is fraud.
It is strange that USCIS would do that. USCIS is not normally concerned with proof of relationship other than proof of meeting within 2 years and intent to marry. The Consulate, on the other hand, is a different story.
Never heard of such. Did they actually say that? An engagement CEREMONY? Could it be seen as a marriage ceremony? Be aware that "too married for a K-1" is a real thing....at the consulate phase.
All you need to have in hand are the 2 year Green Card and the original 48 month extension letter. Once you have those, you are set. No need for anything else at this time.