Due to the forum uh, issues I think the response to this is gone. Fact is that the ABW driver does indeed use cyan, magenta and yellow depending on your tinting mix. At neutral, a tiny amount of cyan is used to cool down the warmth that the 3 blacks in the K3 ink set have. If you run the yellow/magenta tints up it'll also use those inks. Note, I'm pretty sure it's the main colors of cyan, magenta and yellow not the two light versions of cyan and magenta.
There is a real benefit using the ABW mode due to the fact that very little of the colored inks are used and as a result longevity of up to 300 years can be expected with some papers. But the ABW does indeed use some colored inks...
Thanks, Schewe - my reply was eaten and you went into more detail anyway :-)
Regarding which colours are used (light or normal) it's mostly light, but like anything the real answer I suppose is "it depends".
An example, just did a quick A5 sized print on a 9900 with ABW neutral, darker, Premium Luster 250. Inks used:
C 0.03ml, O 0.02ml, Y 0.03ml, LC 0.12ml, MK 0.00ml, PK 0.23ml, VM 0.02ml, LK 0.13ml, G 0.02ml, LLK 0.05ml, VLM 0.16ml
So for C and M, it's the light colours that are used mostly (and O G and Y are minimal, too).