This suggests your system doesn't allow negative utilities, which seems bad because intuitively it's possible for utility to sometimes be negative (eg euthanasia arguments).

It must allow negative numbers, or it's not a group, as (R+,+) is not a group. (Each element must has an inverse which returns that element to the identity element, which for this particular free group is "no one alive".)

However, I believe this specific issue is solved by the lattice structure. If "medium" were "-5" instead of "1", when you add "medium" to any universe, you create a lattice element below the original universe, because we know it is worse than the original universe.