You missed a required entry, because table 164 also applies. It makes no sense for Rect to be required as well as QuadPoints, but that is the rule.
You can either add your Cos obj using the Cos API (into the page structure as needed) or (easier) make a PDAnnot object from your Cos object then use PDPageAddAnnot as usual.
I don't think you need a change notification if you do not change the annot after using PDPageAddAnnot.