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If an adapter is in "NAT" mode, is the "Ethernet adapter VirtualBox Host-Only Network" used at all? From the description in the User Manual is seems like it is not because the VirtualBox networking engine listens for packets. It would seem that these packets are listened for and transmitted out the "main" interface, like "Wireless LAN adapter Wireless Network Connection."

No, "host only" is a different network than NAT. For similar questions please see chapter 6 of the user manual.