I have never heard it called a virtual ground.
Personally I've always called it a 'virtual earth' point, but 'ground' is more normal in the USA than 'earth' - but it's a completely standard opamp term, as others have mentioned long predating IC's.
I have always know it to be called the summing piont were all of the voltages are summed (added ) together.
If I have been doing it wrong all of these years then how come the little mixer I have made works perfectly as you have yet to explain where I was 100% incorrect.
Adding extra unneeded parts doesn't stop it working, it's just a waste. However, for many applications you would have a buffer before the mixer section anyway, but not for any of the reasons you claimed.
I have built all of my own equipment through the years and they have worked flawlessly until I could afford to by a commercial mixing board (mackie 32-8) in which my DIY version still rivals it today (30 years later) as far sound quality goes.
So apparently your terminology may different than mine so there is know reason to insult my intelligence or abilities as a circuit designer.
In the schematic it states pedal so I am assuming Stompboxes.
If you recall (from the 70's) that usaly DPDT (sometimes just a SPDT but improper) switch , Switches the effect in an out of the circuit and when it is switched out the input is directly connected to the output.
Since all of the inputs are tied together should one the Pedals be a delay unit for a slapback type of echo(For an example) it will continue to echo instead of being a single slap back and evenualy start to oscillate profusely if its gain is set high enough.
I am not trying to start any flaming war here I don't know how to explain it any simpler than that.
If you don't understand the bypass switch arrangement then I can put up an example if needed, But I am sure that you do understand its configuration and should not be necessary.
I've no problem with understanding the bypass switching on stompboxes, although I would disagree with the way some manufacturers do it - but I don't see any relevence between that and this simple mixer circuit?.