What's the idea behind the no-feeding-in-the-last-20-km rule?
If you do not race in a UCI  race you do not care about this rule at all. If you are thirsty, you drink. The end of the trip is where you are at the greatest danger of bonking so if you feel hungry or weak, just eat or get some quick sugar.
Actually, the racers can also do so, but they cannot get new food and new water, they only can use what they have.
UCI Rule 18:
&quot;All feeding (from a car and on foot outside of the feeding zones signposted by the organiser) is strictly forbidden: during the 30 first and last 20 kilometres; ...&quot;
If you are just eating your own food or drinking your own water, you are not acting against any imaginary rule on your trips.
I do not think any of the reasons you list is exactly the right one. The closest one would be number 1, but not literally. They are not in full sprint, but disruptions and distractions have to be prevented at some point. And by default that point is 20 km. Sometimes it is postponed when the weather is hot.

Reaction to the update:
I do not think that passing a bottle from a domestique to the leader is forbidden, but I am not sure. It does not appear to be against the wording. I really do think the rule is to prevent the distractions from team cars and from riders moving to the side of the road in the deciding parts of the race. The wording appears to be clearly against external feeding and not against feeding yourself at all.