1- There shouldn't be any difference between using RTW or BI. BI isn't your problem. (By the way it would be v1.5 not v1.3. EB requires 1.5/1.6.

2- The EB team does work on this time period because it is the time period liked by the team. There won't be a BI timeframe EB.

However, if you want a more challenging BI game with some historical accuracy, try out Invasio Barbarorum. And if you like the dark ages try out Arthurian : Total War.