RS422 and RS485; full-duplex or half-duplex?
The RS422 and RS485 standards only specify the electrical requirements. Both can be full-duplex but isn't a requirement of either specification. They are also similar enough that you can interoperate them in some situations.

Now the big difference, which you already noted is that RS485 uses a tri-state system. This means you can have multiple transmitters sharing the single two wire bus, switching individual nodes as needed. In this configuration full-duplex probably isn't necessary or desirable.

In comparison RS422 can only have one driver per wire pair, but the specifications allows for 10 receivers.

If you're only using them for point-to-point communication, there really isn't much difference.