My misunderstanding in the past is that the M channel is equal to mono elements panned in the center. So this is not true because elements panned to the sides also present in the M channel, just without the spacial information.
What I've been taught in the past is that EQing a vocal in the M channel won't affect the guitars panned to the sides at all. So it's not true.
True Stereo is a signal consists of 2 related channels, with correlated material. Not like Pseudo Stereo, which only duplicates a single channel. That's what I mean.
In the 2nd experiment I didn't panned the stereo sounds Hard Left and Right, but in the mid-way. I just wanted to hear how they behave in the M channel.
Thank you for taking the time to have this detailed discussion. I really appreciate it.