I'm not sure I accept that the fatal crash force was "between 3 g and 4.25 g." (page 27 of the AAIB report), I expect a much higher number for a .45 to .75 stop from 55 MIAS? My very modest search for a calculation result suggests as high as 65 G. A friend of mine similarly crashed his 150, similarly stopping in about 12 inches, near straight down. The coroner told me that his stop was in the range of 200 G.
In any case, a nose down vertical arrival will likely be fatal, regardless of seatbelt use.