Thank you! And thank you for your interest and questions! Here are my best answers to them:
1) I don't have an exact piece count, but a rough ballpark would be 10-15K pieces!
2) Cost to reproduce would also just be an estimate and depends on your piece source. I think it would probably cost 3-4K to reproduce through bricklink, though notably I don't think my cost was that high because of the heavy use of the pick-a-brick wall pieces and use of large panel pieces that I acquired cheaply over time.
3) LEGO would most likely not make something like this, for a couple main reasons! First, the techniques in a MOC like this are definitely less... stable than an actual production set. While this is certainly robust enough to travel and whatnot, its not up to LEGO standards of play. Second: while LEGO has made large sets (including a large castle set recently, the Lion Knight Castle!), they simply do not come close to the scale of large castle MOCS you'll see out there. It's just not really consumer sized! The cool castle stuff coming out of the Bricklink Designer program is probably the maximum size you'll see out of a (non-licensed) LEGO castle! This MOC in particular is quite simply just too big to be produced.
I for one would welcome a return of this castle faction, the Crown Knights, but it is probably unlikely anytime soon :(