I've been asking around, tsuwm, and have learned a lot about why Heinlein used pseudonyms, and why he used the ones he did. I have not yet heard from the strongest Heinlein scholar I know, but here's what I've gleaned so far:

John W. Campbell Jr. had a policy of only one story per author per issue of Astounding -- but not necessarily only one story by a given writer. In one issue's "Analytical Laboratory" reader ratings of stories, Heinlein was edged out of first place -- by Anson MacDonald.

Heinlein used his own name for his Future History stories, and MacDonald for fiction not in the Future History that was published in Astounding. He had an informal agreement with Campbell that the Heinlein and MacDonald names would get premium rates, and Heinlein would sell under those two names only to Astounding.

Stories which Campbell rejected were farmed out to other publications under the Lyle Monroe name. Caleb Saunders wrote fantasy, IIRC.

Heinlein also wrote a detective story under the name "Simon York".

"Anson," of course, was Heinlein's middle name, and "Macdonald" was the maiden name of his second wife, Leslyn. Riverside, IIRC, was after the city in California -- he was living in L.A. at the time he was using it. Lyle was his mother's maiden name, and he uses it regularly in early stories, including the protagonist of "If This Goes On" and Mary Jane Lyle, the mother of Valentine Michael Smith. "Caleb" was an Annapolis roommate's name.

There is more to come, I hope.