Slitherine

Reviewing the rules on deployment (pp. 145-46), these exclude ambushes and flank marches from the concept of "on-table troops" and specifically contemplate that ambushers and flank-marchers remain out of the equation when "all visible on-table battle groups have been deployed."

Accordingly, I do not believe that the arrival of a flank march postpones the dismount window (i.e., immediately after all non-ambushing on-table troops have been deployed).

Moreover, the postponement concept is further complicated by stragglers. If an entire flank march straggles, when does the dismount trigger occur? Are troops that do not enter the field considered "deployed"?

On the ambush question, when do you get to know whether your opponent has Battle Wagons? If he does, you have a dismount option, but that won't happen until after you mark ambushes. Can you mark them as dismounted if there is a possibility that you will face BattleWagons (e.g., against Later Hungarians)? If no Battle Wagons materialize, shouldn't the ambush have to spring as mounted? Are you simply unable to dismount ambushers under the Battle Wagon trigger, because the sequencing makes it impossible?

Spike

Are you simply unable to dismount ambushers under the Battle Wagon trigger, because the sequencing makes it impossible?

That's how I read it.