A) I haven't seen him post anywhere his methods. My guess would be that he looked into the average 4th line forward (+13th forward) or 3rd pairing D (+7th D), both of which determined using TOI/GP, and used that as a proxy. However, I don't want to assume anything here.
That does puzzle me (though, if it's both a good evaluator of past performance and a good predictor of future performance, then THoR may be a really promising stat IMO). I would prefer to see an analysis of his tool in comparison to past performance (especially on team level).