1. This is already the case with Tunnel Area. They are not Expressway, but are using `=motorway`. I have suggested to change them to `=trunk` + `motorroad=yes`. For the issue at hand, `=motorway` is a special class that's not defined by function alone. However, it still needs to follow the convention of classification between junctions. It should not suddenly change in the middle, simply because of the Expressway Area limit
2. Airport Rd only leads to North Lantau Hwy. That's why `=motorway_link` was used as a stop-gap. To indicate it is connected to a `=motorway` only. 
3. Tung Wing Rd is a Tunnel Area. Discovery Bay Tunnel is more exceptional, because it is a single carriageway, with no physically raised separation. It also depends on the definition of `=*_link`. Eg  some people find Penny Bay's Highway is only an entrance/exit now, so it has been `=motorway_link`. 