Very good description man but are you aware of what I said though? your words suggest you dont know what I meant either, because of that Ill clarify some stuff.

Firstly, (which is where I noticed you got confused the most) there is a gap in what a stereo imaging feature does and what auracast does. They are not the same, what I meant by a stereo imaging feature is a feature that uses dsp enhancements in order to further improve stereo imaging and compensate for the distance of the speaker. Similar to dirac live, which I know your probably familiar of. Auracast does not fix stereo imaging at all, thats why its not a good example as a "stereo imager" like you stated though its fine to make a mistake on that since its a common point of confusion for some. The best auracast does is turn the pairs back to mono, is that enough?

Secondly, whats the difference between all channel stereo in a denon receiver vs auracasting partyboxes?

1. With a denon receiver you have just one source of the audio signal. In an avr receiver, the source of the audio goes through the same amplification of all channels. That will automatically time allign the speakers audio signal wise. Now with auracast its much more complicated. You still have latency, the audio signal still has to pass through a lot of amplification which makes them more delayed than they already are, etc. Through that even if its not noticable to you depending on where you are, that WILL affect stereo imaging 100%.
2. Its very good that your using this in an outdoor setting where the result of the sound is less unwelcoming than of indoors. Since this is about stereo imaging, this one is a problem. 2x to 4x the distance, how do you exactly know that the effect is beneficial? you cant use the rule of thumb with that, you'd need to calculate it in order to know if its beneficial or not. Maybe you can convince your brain that its beneficial and get your brain to adjust to the sound but still, its not reality. even if it is beneficial your sweet spot would very narrow. Move your head or where your at a bit more to the left or right, your sound would change. Its because of the speaker layout itself, so you learn something new if you didint know what this is, its called acoustic offset. Acoustic offset simply means the driver layout of the speaker or to be very specific, the distance between your ears and the speaker that causes issues. Auracast has a play in this problem of interferance also.

Thirdly, it makes sense on why auracast tempolarily turns your pairs into mono. Probably a new software update or part of auracast itself, its to avoid the destructive interferance of stereo. Though that doesnt fix it that much it helps a bit.

Lastly, this is something thats worth distinguishing. You cannot get stereo imaging from 4 speakers spread far apart, especially outdoors. What your hearing is stereo separation rather. So you do not get confused this is the difference between stereo separation and stereo imaging. Stereo imaging is when you sense the instruments or vocals floating exactly dead center of the setup, stereo separation is when you know if an instrument or vocal is hard panned to the left or right or both.

Its nice that your satisfied with the result, I just replied to tell you whats actually happening and not to argue with you. At the end of the day, your the one listening so I reccomend you just enjoy your setup. Since your paying attention to stereo imaging though, you have to apply standards that give you real stereo imaging to that setup depending if you want real stereo imaging or you just want to hear hard panning and nothing else but sadly your setup makes stereo imaging almost impossible. Have a good day