After 12 hrs sleep, from 1 a.m. to 1 p.m., the following occurs to me which I send to you before I forget, not because it is urgent.

A. The SPIRAL STAIR issue:

1) I would place the 1st and 2nd floor platforms (and the basement exit) exactly as they are in the Addition.

2) I would provide for a deflection inward, i,e, to the west, of such length (6", 12", or  18") of the distal (southern) end of the first floor entrance railing (closest to the entrance) as would permit the door to the outside to open inward as at present.

B. The hardwood FLOORING issue:

1) I would interpret the absence of finished flooring on Shepley's menu as an index: 
a) of the functional inferiority of finished flooring,
b) of customary Nantucket practice  
c) of the availability on Nantucket of tradesmen capable of sanding and finishing flooring.

2) in conjunction of ascertaining Shepley's prices and delivery practices, I would obtain from Shepley the names and phone numbers of tradesmen willing to sand and finish flooring on Nantucket, and ascertain the cost on Nantucket of having this work done.

3) Tim is no altruist. As Mr. Barros wants the painting, Tim might change his mind about sanding and polyurethane.
 
4) You would then have an option:
a) of finished or unfinished flooring, and
b) of the tradesman who would (install and) finish it.