"Is there a daemon running by default that brokers communication between
the local applications like evolution and the local couch? Is this
communication driven through D-BUS.. or is there a local unix socket or tcp
port opened as part of the desktop session?"
desktopcouch runs a couchdb for each user session. It's started by D-Bus
activation, so only running if something uses it.
couchdb is a webservice, so communication is then using HTTP.