var jsons = new[]
{
  @"{""foo"":""fooo"", ""qux"":{ ""bar"":1, ""baz"":0.5}}",
  @"{""foo"":""fooo"", ""qux"":{ ""bar"":2, ""baz"":1.5}}",
  @"{""foo"":""fooo"", ""qux"":{ ""bar"":2, ""baaz"":""2012-04-23T18:25:43.511Z""}}",
};

var flat =
  jsons
      .Select(JObjectVisitor.Flatten)
      .ToDataTable();

flat.Compute("sum([qux.baz])", null).Dump(); // 2
flat.Compute("sum([qux.baz]) < 3", null).Dump(); // True

public class JObjectVisitor
{
    private readonly IDictionary<string, object> _flat;

    private JObjectVisitor(JObject source)
    {
        _flat = new Dictionary<string, object>();

        VisitJObject(source);
    }

    public static IDictionary<string, object> Flatten(string json)
    {
        var obj = JObject.Parse(json);
        return new JObjectVisitor(obj)._flat;
    }

    public static IDictionary<string, object> Flatten(JObject source)
    {
        return new JObjectVisitor(source)._flat;
    }

    private void VisitJObject(JObject source)
    {
        foreach (var item in source)
        {
            switch (item.Value)
            {
                case JObject jObject:
                    VisitJObject(jObject);
                    break;
                case JValue jValue:
                    VisitJValue(jValue);
                    break;
                case JArray jArray:
                    VisitJArray(jArray);
                    break;
            }
        }
    }

    private void VisitJArray(JArray jArray)
    {
        // not used
    }

    private void VisitJValue(JValue jValue)
    {
        _flat[jValue.Path] = jValue.Value;
    }
}

public static DataTable ToDataTable(this IEnumerable<IDictionary<string, object>> source)
{
    return
        source
            .Aggregate(new DataTable(), (current, next) =>
            {
                var newRow = current.NewRow();

                foreach (var item in next.Where(x => !(x.Value is null)))
                {
                    if (!current.Columns.Contains(item.Key))
                    {
                        current.Columns.Add(new DataColumn(item.Key, item.Value.GetType()));
                    }
                    newRow[item.Key] = item.Value;
                }
                current.Rows.Add(newRow);

                return current;
            });
}