So what exactly makes Dawnguard start at Level 10? I looked through some of the more obvious SM event nodes for anything to the effect but found nothing.

Easy, look at the "Change Location Node", there you find the DLC1VQ00Node, there are the conditions setup that lets talk to you, this node refers then to the quest which executes an AI package with the "Force Greet" Archetype, this starts the quest if you go through the dialogues. The Guards are setup in the DLC1VQ00 Quest on the Misc Tab as [Hello] event, with their conditions etc, and the leve requirement itself is stored inside the DLCCQMinLevel GlobalVariable.