In SSiB I don't think he had much oversight.
Actually we have no hard evidence that the making process of SSiB differs from WaHH in any way.
ZUN's statement re: how he didn't give Aki Eda any "instruction on weapon" is vague in itself. For all we know, he could mean that he never told Aki Eda the fan is a weapon until the reveal, not unlike how George Lucas didn't tell anyone Darth Vader is Luke's father before shooting that scene.
I thought that ZUN wrote the mangas. Or was I mistaken?
From what can be gathered (based on ZUN's twitter, Nikenme Radio, and the artists' afterwords), the process involves:
1) He writes the script to a manga chapter. The script includes all dialogue, and instructions on certain details like character expressions. Manga industry people seem to think his writing contains more details than necessary, as when written by a different writer from the artist, manga scripts tend to leave lots of room for the artists to improvise. Of course, they are not all-encompassing; for example, they do not include all sound effects. I doubt ZUN have any exact control on the manga's page layout.
2) He attends an editorial meeting with the manga's editor and artist, generally at a restaraunt. Some preliminary work like designing new characters and other visual elements are done here. It may take several meetings to hammer down all the details on certain special occasions, but mostly it is one meeting for each chapter.
3) ZUN goes home, and falls into radio silence, no longer making any contact with his editor and artist. All he'd do is to wait for the new magazine to be delivered at his door.
As mentioned, the Inaba gag manga is an exception. ZUN only gave its artist Arata general directions such as which characters should appear, and let Arata tell his own jokes.
