How to create a view of hierarchical taxonomy terms and their nodes? I'd like to create an HTML sitemap viewable by a human. I mention that I'd like to keep the hierarchy (tree structure of categories with subcategories) like the following:

- TERM_A
  - Node a
  - Node b
- TERM B
  - TERM_B1
    - Node c
    - Node d
  - TERM_B2
    - Node e
    - Node f


For example, is it possible to display the results as grouped categories and sub-categories, by managing the order or results (relate to taxonomy term, sort by term weight and nesting level)?
You can do it using the grouping fields feature inside Views. 

Create Views listing your nodes (titles), add a Field showing terms they're tagged with (sub-term). 

Then add a Relationship for that same Field (the Taxonomy term reference Field). Add another Relationship to introduce Parent term, use the first Relationship here. 

Now add another Term name Field and set it to use the second Relationship, this way the Field will show the first level Taxonomy hierarchy for a node. 

Now edit your Format settings and set the Grouping field Nr.1 to use the Parent Field, and set the Grouping field Nr.2 the usual (sub-term) Field. 

If you need further control of the output you can overwrite to change HTML, and use CSS to emphasize the hierarchy visually.