Think of this scenario: I ask "is everything I am doing the optimal for my subjective preferences?" Now think at your question. It is provable that the oracle answer yes (or no) to my question if and only if the oracle answer yes (or no) to your question, and vice-versa. This make my question a better choice since it is less complex (less bits). If you try to schematize some possible cases, you will see that the answer of the oracle in my example and yours is always the same.
What do you mean by "where the motivation comes from"?
The most complex system is the one that can generate complex system itself, outside of biological reproduction. Based on this definition, human beings are the most complex biological systems that we know, even if it sound too anthropocentric.
A couple of points to your points.
1) Bell's inequalities only seem to disprove local realism. In addition, many scientists criticize the assumptions and, in any case, they are compatible with different deterministic systems or with non-local hidden variables, so it is far from being a definitive tool to prove or disprove determinism.
2) If you take determinism in the broad sense as the main subject of the discourse, then eternalism is a subset of determinism and not vice versa. Other subsets may be: Hard determinism, MWI, Biological determinism, Theological determinism and many others.
how would you change the question between the two cases?