Understanding resistors on inputs/outputs of op amps
Regarding R10, the virtual ground is driving stray capacitance. If you loaded the output of U2 directly with that capacitance it would probably oscillate. 

R8 actually hurts stability of U4. It would improve it only if C4 was connected to pin 1, however in most cases users will not load the output capacitively (eg. by attaching a long cable) so it's probably meant to limit the short-circuit current from the amplifier, and keep it from driving the virtual ground too far. 

R12 limits current in case of a fault such as a large voltage applied to the input- he uses 270R to reduce the number of different part values- but a larger value might be appropriate too. Also perhaps it acts as a jumper for his single-sided layout. ;-)