Also you mention the wellhead that Sargent painted Hunt standing next too might not have yet been setup or even shipped down from NYC since the final flurry of construction was going on.  However George Vanderbilt did own a 13th century unpolished Italian marble wellhead that is on the site today. 

It is also certain that the wellhead in the Sargent painting is nearly if not completely identical to the wellhead on the property today.   See the photo of me taken in a Hunt pose in the fall of 1998.  it is my belief that the wellhead in the painting is that very one still on the property. 

 
Richard Morris Hunt   
1895  

Note 
 

  
Photo of a Venetian well-head 

• See There were 6 total paintings owned by Biltmore estate