Have you setup the Rpi as an AP as Frank explained? (page 2 of this discussion)
No, I found this thread after I managed to create an ap with the create_ap script, like specified in my previous post.
EDIT : Ok, I set up my AccessPoint like Frank explained, it works very well and the explanation is very easy to follow and to apply, great job. It doesn't change a thing on my other problems though.
Can you see the Runeaudio UPNP renderer?
Nope, with BubbleUPnP I can't see it.
the Airplay renderer? (using Airstream or Airaudio or equivalents)
Yes, I can see the Airplay renderer
While on your eth0:
Can you see the Rune UPNP renderer if you search for it?
No, but if I specify IP address and port I have a response from the device. However it is a HTTP 500 Error.