One question you asked in there was how was being able to "unwrap" a start key into a Process key given the Process creator useful. The big use is authentication. For example, the KeySet class has a couple of methods that take other keysets as arguments (i.e. RemoveKeysInSet). It uses its process creator to verify that the passed in key is authentic, and then starts an internal protocol with the other keyset.
Being the writer of the KeySet class, I am biased, but I think it provides a
good example of how Eros allows code to be structured differently. Take a look
at the second long comment in:
http://www.eros-os.org/eros-src/domain/keyset/keyset.c for a description of how the protocol works. It also uses co-routine invocation (CALL on a resume key).
(uh, looking over the code, the first comment is somewhat out of date...)