I don’t think these assertions are compatible. If utility is subjective, it means that you can’t assign a cardinal number to quantify it. You can’t say that Bob derives three units of happiness from eating a bar of chocolate and Jane derives five units and Joe only derives two units.
You are assuming that it is possible to assign cardinal numbers to preferences, because you need cardinal numbers in order to infer the existence of a sum ‘happiness number’ of all the people in the village and their wants subtracted from the hypothetical sum utility total of all the millions of people who would putatively gain from the proposed hydroelectric project. In order for your assertion of “clear” utilitarian benefit to be correct, utility cannot be subjective, and yet you maintain that it is.
What one can do to maximise utility, if utility is subjective, is use ordinal numbers to rank someone’s preferences, if you allow them to demonstrate their preferences without interfering. If you give someone $5 as an unconditional gift, to set up an example, whatever they choose to do with it will demonstrate which of their preferences, regarding that money, is superior to whatever others they might have at the time. They will have a choice between saving the money, or spending it on whatever they like, and if they decide to spend it on chocolate, you know that this is what they preferred to all other options they perceived as available to them at the time.
In order to maximise everyone’s subjective utility, you have to abandon the initiation of force in absolutely all cases, because people cannot demonstrate their preferences if they are being coerced into assuming other people’s preferences. If utility is subjective, meaning that you can’t assign people cardinal ‘happiness numbers’ and then add them all up in contrived groups and subtract that total from the total of a second contrived group, then the non-aggression principle must be adhered to absolutely in order to maximise utility.
Are you an arithmetical absolutist if you think that 2+2 always =4? You can only dodge on that point if you decide that the symbols used to represent the numbers actually mean something else in your mind. Of course Arabic numerals, like any other symbols, are conventional. Try to see beyond the symbols, to the immutable logical principles they are supposed to represent. If everyone could do that consistently, the world would be a better place.
You will probably find that the Brazillian state actually owns the land in terms of civil law. They claim almost the entire Amazon river basin because the King of Portugal once claimed that God gave it to him. Seriously, and these are the people who criticise us for antiquated thinking…