Peter, I think you're seeing Graff's roots as a Caniff-Sickles school artist. I've read a bit about him. An ongoing struggle with alcoholism interfered with his producing X-9. According to what I've read, Paul Norris frequently did fill-ins and ghost wrote/drew at least one full story per year during much of Graff's tenure. I haven't run across names of any other ghosts or assistants.
I used to brag I could spot Paul Norris anywhere. But on X-9 he absorbed Graff's style so completely that I can't sort out which strips are his. In my opinion--and it's just an opinion!--Graff did these stories. The layouts, the lovingly-drawn woodsy backgrounds, and the bravura use of Craftint (especially in the snowstorm sequence) look like his work. The story is also Graff's style--his stories were always strong on character interaction and human interest. Inspired perhaps by the stories of friend and ex-studio mate Caniff.
It's a shame Graff isn't better appreciated. As time went on his characters took on an unpleasant Dick Tracy look which I don't like, but on the whole his drawing and scripting were well above the average for a newspaper strip.
Link to the book:
Secret Agent X-9 1951.10.18-1952.02.23