Painting on top of clear topcoat
As for the paint compatibility, the standard advice is to either paint an innocuous area and try it, or do a crosshatch test.

The low sheen is caused by the paint being intentionally "bumpy". That tends to produce good binding to the next coat, so I would expect reasonable results.  

The other reason we use primer is as a "barrier coat" to assure chemical compatibility (that the paints don't react to each other or fail to stick).  Again a paint or crosshatch test will reveal that.