Disclaimer upfront: I am not an economist.
I’m not sure it follows from the assertion that markets (while Pareto optimal) are not “optimal”, that they are necessarily zero sum. Regardless how you would define optimality, it’d mean the sum could be larger, but that doesn’t require it to be zero.

The notion of optimality itself is fraught. How would you *objectively* establish a share of the economic surplus between the producer and the consumer that is optimum? Say I have widget for sale, and my WTA is $2, and you are interested in buy8ng one and your WTP is $3. We can have a non-zero-sum transaction at any sale price between $2 and $3. What is the optimum price?

The idea of positing a “common good” mental account is interesting, but don’t we have a whole array of mental accounts anyway? If you spend $3 on a coffee, or $3 on a book, or give $3 to charity—do they deliver exactly the same value… or do we put each expenditure in different mental accounts, that we don’t compare with each other?