There are rather concrete problems that strictly prevent it from being possible to efficiently map graph (object) database access patterns to a relational database.

Do you mind going more into that? Naively, it seems like prolog/datalog describe graphs pretty well and they're inherently relational. Relational databases have typically just optimized for row-oriented OLTP uses instead of columnar OLAP, but there's nothing inherent preventing one or the other. They're duals of each other.