David,
I agree with your observations about the differences between the coach and caboose #4.
From what I have been told today by some that are familiar with the history of the coach, I conclude that the builder of the coach, built it as a completely freelance design, following general prototype practice, and that it was built as entirely new construction except that he did incorporate some of the iron parts that he salvaged from Q&TL caboose #4 during the period it was sitting in the farmyard. However, I would not conclude that all of the iron parts on the coach came from caboose #4.
In that link I posted above, there are two photos of caboose #4, and I also have an accurate drawing of the caboose that was made by measuring the remains of it in the farmyard. The drawing shows passenger car trucks with journal boxes in pedestals with equalizers. But they are almost entirely different in detail that the trucks on the coach in Wisconsin.
Caboose #4 was made by shortening a 3-foot-gage coach that I believe was acquired by the Q&TL from the Mineral Range RR. In looking at the photos of caboose #4, and the drawing of it, I see nothing in the design of #4 that matches this coach in Wisconsin. The roof design, clearstory windows, side windows, all window trim and framing, outside wood paneling on sides and ends, door panel detail, trucks, end platforms—all of these details are different from caboose #4. So this would tend to reinforce the conclusion that the woodwork of the car in Wisconsin is all new construction rather than incorporating wooden parts from caboose #4.