But I still have one question that I can't figure out - when you have that Important NPC (say, the Big Bad for a multi-session campaign) that has his or her own topic, where should the statistics for that NPC reside? Is there a suggested practice for that?
Pretty much what @kbs666 suggested. You place both the HL Statblock and Portfolio snippets into the NPC topic. Then you identify the NPC topic in the in the scene or other place where it needs to be referenced and you hook it up there. In some cases, it may also make sense to duplicate the portfolio and statblocks snippets in the scene.
Hope this helps!