If the XH A1 is used as wide as possible with the LETUS lens, and a Nikon 24/2.8 lens is used - is the result actually wider than with just the XH A1 prime lens alone? Whatever the result, can someone estimate the equivalent 35mm lens it is equal to?

Maybe it is 50mm since it is what is depicted as a normal eye vision will see. The wide angle lenses tend to add vignetting and warped edges as you get smaller in mm. IE 16mm fisheye lenses.

Actually about 42-43mm is actual human eyes, but 50 is closest if not considering that nikon 45mm pancake lens.