“The question remains, why those two extra categories of dukkha (dukkha dukkha, viparinama dukkha)?”
I am not sure why you say that because “dukkha” can mean either “actual suffering” or the “causes for suffering,” as I explained above.
– Dukkha dukkha and viparinama dukkha are in the first category.
– Sankhara dukkha is included in both categories.
“The world is made by sankhara/sankata of uncountable beings, thus a single being will experience bad vipaka based on the result of all sankhara (which we call “planet earth”).”
Each one reaps the fruits of their kamma, especially regarding rebirths.
– Of course, the world is made due to the “collective kamma” of all beings.
– Also, there can be some “unaccounted suffering” due to things like floods, earthquakes, etc. But those also arise BECAUSE one is born into that existence.
– But rebirths are ALWAYS due to one’s kamma.