CSV to WordPress posts: do I have to populate each possible field?

Nope, the WP is pretty flexible about this and will perfectly handling omitted fields. Including assigning IDs (which is actually best left to it, outside of edge cases when exact IDs must be imported).

The only situation that natively won’t let you create post is if neither of title/content/excerpt is provided and post type supports any of them. Even then there is wp_insert_post_empty_content filter available to adjust the behavior.