Yes, you are correct. Henry Daniell's part in "Castle in the Desert" was fairly complicated: Watson King (also known as Jim; alias Cesare Borgia). As Watson King, he must have actually been something of an accomplished artist!
In the end, the most important connection was the fact that he was Lucy Manderley's long-lost step-brother, Cesare, and, of course, he plays a "very important part" in a murder!
Again, thank you, Marsh for noting this interesting detail from the final 20th Century-Fox Charlie Chan adventure!