The power is from a 9v wall wart, regulated with a LM7809 (IC1).
No.
Again, the LM7809 with a series diode needs an input of 12VDC.
1) Biasing non inverting inputs for single supply opamps: What are the 'rules' for determining the values for the resistors and caps? The 100 nf cap i understand is for transients but how big (or small) must the larger one be? Resistors, why 10K or 100K?
Each opamp has a different amount of input bias current that is supplied by the resistors. The TL074 has Jfet input transistors so its input bias current is almost zero and almost any resistor values can be used. 100k instead of 10k is fine. The 100nF bias filter capacitor filters out hiss from the voltage regulator IC and should be 3.3uF to 10uF.
2) Can i use one voltage divider / cap network so supply the vcc/2 bias for TWO opamps (on the same side of chip and the transient cap between pins) or should they really have each their own dividers and caps.
Yes, only one VCC/2 circuit is needed since the input bias current of the TL074 opamps is almost zero.
3) Output cap size: Is the frequency formula here the good old 1/(2*pi*R*C) where R would be the expected input impedance of the equipment we are plugging the mixer into? I believe that is normally 10k?
Correct.
The TL074 and most other high frequency audio opamps oscillate when they directly feed the capacitance of a shielded cable. Add 100 ohms in series with the output from C7 to prevent the oscillation.
4) I've often read that 9v wall warts give 12v unregulated. With diode D1 (1n914) in the path, is this reliable enough for the LM7809? I guess i'll see tomorrow...
I have a small "9V" wall wart that has 18.5V with no load but is 9V with its rated load. I have another 9V wall wart that has an output that is always 9V which is too low for your circuit.
Use a wall wart that is rated at 12VDC at 500mA.
EDIT 5) I've sometimes seen a small cap (27pf - 47pf) in parallel with the feedback resistor in inverting opamps (R5 in my schematic). I presume this is to filter out very high freq. Is this desirable and if so what is the formula to determine the cap value?
The extra tiny capacitor is not needed unless you have a radio station transmitter next door (then other capacitors are also needed).