Namespaces in R packages

I have a start on an answer on the devtools wiki: http://adv-r.had.co.nz/Namespaces.html

Few years later here....

I consolidated findings from Chambers, other StackOverflow posts, and lots of tinkering in R: http://blog.obeautifulcode.com/R/How-R-Searches-And-Finds-Stuff/

This is less about implementing NAMESPACE/IMPORTS/DEPENDS and more about the purpose of these structures. Answers some of your questions.

The clearest explanation I've read is in John Chambers' Software for Data Analysis: Programming with R, page 103. I don't know of any free online explanations that are better than what you've already found in the R Extensions manual.