(a) Subjectivity: The “scores” on any of these dimensions is pretty complex because each of of these traits have sub-traits. For example, extraversion has sub-traits such as activity, sociability, shyness and so on. Even if we disregard the correlation between traits and sub-traits, it is still difficult to know impact of trait/sub-trait to the whole personality. In short, yes, the scores would be heavily subjective.
(b) Homogeneous personalities: I do not have any data to prove or disprove. But, since the genetic makeup can’t be changed in a human, a majority of the personality can’t be changed easily. Also, we can only optimise to a certain extent. If a person is low on say, agreeableness, we could make little more agreeable than what he/she is. Making them a yes-man or yes-woman would still be close to impossible.
But then, my guess is only as good as yours.