You confuse me. Router forwarded on port 4042, but router configuration of 6346. I'd suggest both should be the same. To connect to the gnutella network, you don't have to use port 6346, that's just the official gnutella port.

I think you meant to say in your 2nd paragraph that client C cannot direct connect to the others, right?

If all is ok, then the ip should show above the direct connect search box. If it's not there, then there's a firewall problem. Does client C show the brick wall icon at the bottom of the window?

Also with direct connecting, you need to have available bandwidth & slots for both up & down.

Out of curiosity, has client C tried sending a magnet link to the others to see if they can download from it?

Also, I've heard it said that it's better to do a separate port forward for each of UDP & TCP (using same port no.#). That's how I've done mine.