The XIN displays OK in the style and named symbology managers. Although it might be missing some features styles as explained below...
This is clearly related to the corrupt FeatureCategoryCollection. There should be a 1:1 matching of Feature Style Names to the list in the Root of the Feature Category. The FeatureCategoryCollection is only visible in some type of editor configured for XIN files.
I have worked for three different companies with XIN files from multiple sources and found this issue often enough to find it necessary to create an XML Report Stylesheet that creates a new FeatureCategoryCollection section which can be used to copy and paste into an XIN to re-synchronize the two lists. In fact, I've created it twice because one company retained the rights to any stylesheets I developed, so I had to created again from memory once I landed at this company.
Apparently when linking an XIN, only the features in both lists will show up in the link. In this latest episode, there were hundreds of feature styles missing until I corrected the issue. And none were showing up in the project explorer nor in the styles menus. But many were missing in the Style Manager. I have other XIN XML Reports and when I looked at their output compared to the Style Manager, there were many in the report that were not in the style manager.