For the front section you've described in part #1, I calculate 26 x 32 x 10.34 = 8602in3. I calculated the rear section you've described in part #2 as two extruded triangles and an extruded rectangle. I combine the triangles to form a rectangle. The two triangular portions are 6 x 6 x 32 = 1152in3. The rectangular portion is 6 x 14 x 32 = 2688in3. Combined, it is 12442in3 or 7.2ft3.

The horn, driver, panel wood thickness, braces and brackets all displace volume inside and so must be removed from this figure. I don't recall the displacements right off hand but I remember they were around two cubic feet, and virtual volume increase from insulation adds some of this back. Fortunately, at this size of cabinet, a few hundred cubic inches difference either way doesn't make any difference.

As for resonant frequency, I don't know about the accuracy of WinISP. I use the Helmoltz formula to calculate resonant frequency: