There's not really any best practice around this so really up to the mappers preference. For me if the refuge island is long I might map it as footway=traffic_island, but if it's only 1 or 2 meters I would just use a node with crossing:island=yes.

If the shared paths don't have markings how do we know it's a shared path? Sorry I'm not from WA so I don't know how it works, but https://wiki.openstreetmap.org/wiki/Australian_Tagging_Guidelines/Cycling_and_Foot_Paths#Footpath_Cycling indicates you don't have the same footpath cycling restrictions we have in NSW, so would be permitted.

Usually if bicycles are explicitly signposted or marked it would be bicycle=designated but if bicycles are allowed but not explicitly signed or marked then bicycle=yes, but again I don't have the local knowledge so take this with caution.