Okay, thanks for feedback and testing.
Re-reading the above posts again, I am still not entirely sure if ZNC is relaying messages or playing them back to the status window 2 connection. If ZNC is relaying messages, it should not be sending echoes to the status window 2 connection, since the status window 2 connection has no idea about which messages were sent and is therefore not expecting echoes. If ZNC is playing back messages to the status window 2 connection, so that it shows the same channel/query window contents as the status window 1 connection, then the behaviour seems fine.
At the end of the day, all mIRC can do is implement the IRCv3 spec for echo-message in a way that works on ircds that support it.
In the case of ZNC, its implementation of features like playback/self-message/etc. have resulted in behaviours that aren't quite standard, which is what we may be seeing with its echo-message support currently.