As you already noticed NodeInstanceLog table keeps only information relevant to the runtime engine which means only on node level - and not the details of each node (in case of human task - work item).
the relationship between runtime engine and task service is via work item id. Unfortunately it seems it is not stored anywhere after task(work item) is completed. It looks like a good feature request to be added so runtime data and task data can be matched later on too. Please file a jira issue for this.
And as workaround (potential contribution as well) is to extend the current logger to save work item id as part of the nodeinstancelog if present.
HTH