1) Craig...... Yes, your reasoning is correct. Since the appliance was manufactured using a hardwired 3-prong cord supplied by the manufacturer, any alterations to the unit will void the UL listing.
2) My interpretation of Article 422.16 states "installation instructions of the appliance manufacturer"
Since the manufacturer installation instructions only recommend but DO NOT require a dedicated circuit, I see no problem in installing a 3-prong grounded duplex in the upper cabinet and connecting it to the existing circuit as per your previous comments.
Basically what Article 422.16 is stating is that a dedicated circuit for a range hood is required unless stated otherwise from the manufacturer. Since Whirlpool does not state or require a dedicated circuit and they only make recommendations for one, I see the Whirlpool dedicated circuit as an option for the installer. Code states to adhere to the "manufacturer installation instructions". In this range hood model, the manufacturer of the appliance is over-ruling the NEC (number 5) and only uses the verbiage "Recommended". They never use the word "Required".
I've seen many electrical appliances such as microwave ovens where the manufacturer specifically states within the electrical requirements that dedicated circuit is required. A recommendation is different from a requirement. As I mentioned in my previous reply, I've never come across a standalone range hood that had a dedicated circuit installed unless it was a microwave combo. Last week, I installed a combo microwave/hood where GE was the manufacturer and it stated in that particular model that a dedicated 20 amp circuit was required.
Many manufacturers will use the word "recommended" to cover themselves in the event that the connected appliance becomes overloaded and trips a breaker due to other loads that are supplied by the same circuit. If the existing circuit is only a few lights and the other room receptacles will not provide power to such loads as a portable heater or high wattage appliances, I do not see any need for a dedicated circuit on your installation.