Perhaps this is expected behavior, but I had a group radar equipped during a voy escort, and got no radar extension in storms. Orne guardians were not visible on radar nor targetable until a few 100 meters, and I saw no wireframe sweeps ever. Does the shared extender only work with player groups? Does is not function when flying solo? Or has a recent update killed its functionality and gone unnoticed because people don't use them much?

Sounds like you are confusing the radar extender with shared group radar. If you have the shared radar equipped it relays targets on your radar to your group members' radar.

I suppose the extender bit in the name of the 'Shared Group Radar Extender' is misleading.

I think the issue here is that CD expects to not only broadcast her OWN radar contacts to her group, but that she would also receive the rest of her groups... This is one of the reason no one uses the damn thing, as it doesnt actually help much.

The Shared Group Radar works for you when you are within the radar range of the player with the "Shared Group Radar Extender". Storms and fog do reduce the effective range of it.