Not sure how helpful my "input" would be, but here it is.
I've done quick test
on my PW3 (so, not the same device I know) with firmware 5.9.7 (originally jailbroken while on fw 5.6.5, RP + CRP installed at the same time ; newer versions RP & CRP bin installers downloaded and installed sometimes in last 2 months).
Main menu in diagnostics + "reboot or disable diagnostics" sub-menu.
In order to get to option to enable USBNetwork you should choose: Misc individual diagnostics > Utilities > Enable USBnet
But in my test case the USBnetwork was
already enabled (tested on Windows 10 with WinSCP) when I've initially booted into diagnostics (I remember that not being the case on my PW1 whose diagnostics menu option looked totally different)
The last 2 photos just to demonstrate that upon taping on "<EXIT DISABLE> Disable diagnostics" menu option I've also had that "Please use serial port" screen showed.
But it went away after ~2s without any extra actions taken by me... that screen was followed by couple of screen flashes with "boy under the tree" image before that last screen (that shows output from screensaver hack script) showed and Kindle resumed booting into "main" / normal OS. The USB cable was of course disconnected from my Kindle once I've entered the diagnostics and after I've tested USBNetwork functionality.
