A1:
I'm not sure but I think the forcing is like binding:
If you are connected to the Internet AND to a LAN you have
to addresses: One for the LAN and one for the Internet.
Now it can happen that LimeWire binds itself to the LAN IP,
which is not working in the Internet. By forcing you can set
the IP LimeWire should bind itself to and where it should listen
for incoming connections.

A2:
The gnutella network needs your IP.
Without it you wouldnt get connected.
So in Gnutella its not possible to disguise your true IP.
(Maybe thru Proxies, but I havent tried that)