I had previously only used the headphones with Bluetooth. I have now tested the operation with audio cable. If the headphones are switched off, they can be connected to an audio device (3.5 mm jack required) and they work like a normal headphones. However, the quality is reduced. It is hardly recommended to use it like this.
If it is switched on and you plug the audio cable (2.5mm plug) into the KH, Bluetooth is switched off. A voice confirmation confirms this. The KH is also no longer displayed in the Bose app.
The audio quality is now as good as in Bluetooth mode. I can change the volume on the KH using the buttons. I can also switch between ambient mode (with noise cancelling) and noise cancelling mode.
So it is practical in flight mode. Can also be useful for people who cannot tolerate Bluetooth radiation.
Works perfectly for me and without any further voice announcements from the KH.
Tested with
Bose QuietComfort 45 - Firmware 2.0.4-3475+32e4b0e .

In principle, yes, but the headphones then say "ready to connect" every few seconds.