I'm a little confused by what you are saying. Let's see if we can figure this out.
"Whereas it appears you have entered [unknown] as the name."
No, I did not add [unknown] as the name. If I did, I would include it in the Index. This .jpg is showing FTM 2012 where "Shared Facts with [unknown]" is the labeling used at the time.
"In your example there is actually a listed shared fact with the unknown which is different from the Original Poster aresquare's situation where there was no shared fact nor any media or source."
In my example, all I would need to do is delete the Marriage fact, and I would have what aresquare has. Perhaps there was originally a Marriage fact that got deleted. Regardless of whether there is a marriage fact or not, the shared Relationship is Spouse (not marriage), and is unaffected by the specific fact, and should appear in the list of families that can be detached from Alice. I checked FTM 2017, and although the graphics are different, the unknown spouse appears in the detach list and is easily detached. Of course if there are children, leave things as they are, and name the unknown spouse. See the FTM 2012 .jpg and the FTM 2017 .jpg.
If I have misunderstood what you are saying, please let me know.