There is certainly some logic to your diagnosis. What usually happens is that the Starter won't engage in Park and/or Neutral if the Neutral Safety Switch (NSS) is bad. My understanding is that the Starter Solenoid is energised from the START position of the key switch and that passes through the NSS so that you cannot start it in gear. I supose that if the NSS was shorting out you might get power from Reverse Light circuit to engage the starter solenoid. That would certainly blow an 8 amp fuse but maybe not a 16 amp.
By jove I think you have cracked it Holmes!!

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1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown