But isn't the attacker doing something fundamentally different--ie, not trying to "join" with me, so therefore not aiki? In fact, if the shomen-uchi is made with intent even at a slow speed, then its purpose is to make my one body into two parts--to split me right down the middle. Or am i missing something?
No, this isn't quite on target... The point of bringing up this book and the topic of body mapping was that it has broad implications for motor learning and how we train. This isn't something unique to "aiki", it's how everyone's brain works and how it organizes your motor skills and a bunch of other stuff as well.
The reason I brought up "aiki" was that various really high level teachers from a number of arts, not just Aikido, have been able to do things to me that I couldn't really understand. I can now do things myself that I really couldn't explain, I could just do them. The topic of "body mapping" allowed me to see the "why" of these things. It really doesn't help at all with the "how".
And finally, it is a total misunderstanding of what is "aiki" to think that one can't use "aiki" in an attack. "Aiki" is value neutral. It could be used for evil purpose, it can be used to attack, it has no connotation of non-violence, or peacefulness in and of itself. If two warriors trained in "aiki" were to fight they would each be using these principles and the one who was best at it would win. But this is an entirely separate issue from a discussion of body mapping as a neurological process.