https://www.freedesktop.org/wiki/Software/PulseAudio/Documentation/User/DefaultDevice/ - default is not the same as fallback in PulseAudio. One can't really set a "default" device as described in the link within PulseAudio itself.
The "correct" way to do it would be to define a remote sink via module-tunnel-sink as you are trying to do. You say it is not possible on that version of PulseAudio, and then I can only recommend you to recheck syntax or upgrade in some way. Setting PULSE_SERVER can be done for login shells as in Gilles's answer. Below I write explicitly about my solution (with a recent PulseAudio version).
My exact lines from /etc/pulse/default.pa:
.ifexists module-tunnel-sink.so.nofailload-module module-tunnel-sink server=192.168.0.1 sink=alsa_output.pci-0000_00_1b.0.analog-stereo sink_name=d-f05.fail.endif...just as you try to do. If this works, then you can set a "true default" via e.g. gnome-volume-control, where your aliased sink should show up as an available output device.
This way makes you able to switch output on the fly for all applications.
The definition of the remote sink can also be done nicer via Avahi, but I haven't been able to get that to work here just yet (not a PulseAudio problem).
