> Why are important quantities such as 2.2) and 3) estimated per person when their link to population size is very unclear?
The answer for 2.2) seems to be that this water would be mostly related to concrete production for habitat construction/maintenance. More people means more construction/maintenance.
See:
"Concrete production will most likely use the most water out of all the manufactured materials because that water isn’t easily reclaimed. And, although there may be more need for manufactured products towards the early stages of the colony, the amount required will be normalized to the amount of people in the colony."
The explanation for 3) appears to be that this water is a byproduct of oxygen production, which occurs in proportion to oxygen consumption, which should be proportional to population size.
See:
"This amount relates to the water that would be extracted from the atmosphere during the process of extracting the required oxygen for breathable air (Wieland, 1998)."