You are right about the identifiers. And that is exactly what makes us concerned about access patterns attacks. They are not as dangerous when queries are rare

You are right about the identifiers. And that is exactly what makes us concerned about access patterns attacks. They are not as dangerous when queries are rare. But we do explore possibilities to mitigate this attack vector completely using oblivious RAMs which become more and more performance. You are also right about clients updating the b-trees. Currently we use object-level locks to keep data consistent. We may add storing deltas for buckets to keep them on the server when a conflict appears plus use additively homomorphic encryption for counters (BTree.Length object).