Your question is a bit confusing.
What is the proper way to represent in popular press the status of a
commissioned officer of the United States Marine Corps. who is not
retired, has a continuing service commitment, but is no longer active
This seems to indicate a person who still has a commitment to the Corps, but is not active duty. That would be a reserve, and they are still members and retain their rank.
However this sentence:
Unlike all other branches of service Marine officers are commissioned,
by both custom and law, for life.
makes it sound like you're talking about someone who has stopped serving, but because of some special USMC rule haven't "retired" per se.
USMC servicemen do not retain rank when retiring in terms of authority. They are encouraged to use the "Once a Marine, Always a Marine" mindset; they're often accorded the personal respect their former title held and are not discouraged from using their old rank for formal stylizing - but they're certainly not Marines in the eyes of the government.
If you're looking for a stylistic guideline for writing, check this out: Veteran Marines. It's not an absolute standard that everyone would agree with, but it's an acceptable reference point.