[Basic/MA] Defense On Horseback - Seated?

[Gigermann]

Recently I ruled that someone on horseback is considered "Seated" for Active Defense purposes (-2). But there is also the Riding skill-based penalty (B398) which I later considered might override that Posture. So, the question is: do they stack? It's not clearly stated either way, as far as I've been able to determine. Have I missed anything?

Of course, if the Active Defense penalty applies for being Seated, it stands to reason that the penalty to be targeted for that Posture might also apply.

Given that this question arose from a group of mounted bandits riding down a Heroic Archer PC, I also began to wonder about other defensive situations like "riding evasively"—which is apparently not mentioned either. Logically, you could apply a Vehicular Dodge (B470) based on Riding, but as written, that only applies to attacks on the vehicle (or mount, in this case).

And lastly, aside from All-Out Defense on the rider's part, are there any other defense options? Can't Retreat. Extra-Effort would apply (but our group has a standing rule that NPCs don't get to use that except in special cases). Equine Acrobatics, maybe…