Dongshan is considered the root of the 'Soto' line of Zen. In fact, his name gives us the Chinese Character "to" in "Soto".
Now, a key into this thorny maze might be found near the end of Master Keizan's talk (the bottom of pg. 197 in Cook). How do the insentient and the sentient preach the Dharma, and how can the preaching of the insentient be heard? (Let me paraphrase a bit.) "Within us, when the divisions of ordinary and sacred do not arise or cease in the least way ... nor thoughts of sentient or insentient ... there is heard a subtle consciousness that sweeps away being or nonbeing, and is keenly aware." ... then all things accord with true reality, all things sentient and insentient preach the Dharma.
Hixon takes up this same theme and the opening Koan, perhaps in a way that makes it clearer than the presentation of Master Keizan in Cook.
Perhaps, when we stop listening with merely the 'ears', we can begin to hear the nonsentient preaching of the Dharma.
And ... on the part about his mother! Well, what can I say beyond that it was a different time with different standards. In those days, "homeleaving" meant just that, and cutting off ties to society and home was so much valued. Mom even thanked him for letting her die of a broken heart .... or so he dreamed (was it just his guilty conscious?). Perhaps his seeming coldness was actually a beautiful sacrifice on his part, and a gift to his mother. That's what Keizan seems to say.
On the other hand though, perhaps Dongshan was so wise about some things, but still a jerk as a son?! He would not be the first brilliant Zen teacher to make some stupid, selfish choices in one part of his/her life (see several recent examples). Frankly, seems Dongshan was pretty "heartless", no matter his subtle understanding of the "Heart Sutra". He could hear the "sentient beings" preaching the Dharma, but not the cries of his own mother!
Cook from 193
Hixon from 179