Of course, I cannot answer your question, but I just wanted to contribute this thought.
I do think that some other people (besides me) might find your revised sentence a bit ambiguous.
That is to say, some of us might think that he stands the chance of losing four people: his extraordinary partner ("Mr. Smith," for example), his gifted wife ("Mona," for example), and his
two brilliant children.
By using "in," the author makes it plain that "an extraordinary partner" refers to his "gifted wife." That is to say, "He stands the chance of losing an extraordinary partner in [the person of] his gifted wife."
P.S. If you don't want to use "in," maybe you could say something like:
"He stands a chance of losing his gifted wife, who is an extraordinary partner, and three creative and promising children."
Student or Learner