I too would be happy if someone could answer your two last questions:
Why do the pins have the suffix _RESUME, and is this indeed the proper location to interface the SMBus (although I can't say I have found any alternatives)?
Btw. It would be nice to know if the SMBus signals are inverted or not.
Doesn't help much, but have a glance at the Custom Solutions Header for Desktop Boards page:
Towards the bottom of the page is this table for the DN2800MT
Key (no pin)