Thanks, all.
The "ambushers as reinforcements" was really the most tricky part of the situation for me and I appreciate the ideas and real-world experience. The wording does leave some room for interpretation (ambiguity in a GW rule set…say it isn't so!). As noted, if they are reinforcing then the ambusher's aren't "deployed" at the beginning. Looking at the book last night, reinforcements are specifically "deployed" when they are rolled successfully and not before. But in the case of an ambusher this just doesn't seem appropriate that you are allowed to pick your Terrain a couple turns into the game.
I think the spirit and wording of the rules both allow for what you describe above: Assign the terrain at the beginning of the game, during initial deployment, for each ambusher regardless of them being reinforcements or not. Roll for the ambushers that are reinforcing as normal reinforcements, but follow all other ambush rules (can't reveal until after turn 2, for example).
One thing I forgot to check was if rolling for reinforcements happens at the beginning or end of your Move phase. Revealing ambushers happens at the beginning, so if rolling for reinforcements happens at the end then it would force an extra turn as well. I think this is a fine limitation tbh considering that combining ambushers (which you don't intend on revealing for a couple turns anyway) with reinforcements (which are units you can't use right away) lets you lessen the impact of reinforcements on the strength of your force.
I know anyone with a strong Ambush contingent would probably want to interpret the rules to say that they don't have to assign the Ambusher's to a specific terrain feature until they are successfully rolled for, but I think that's another situation of trying to manipulate a gray area to maximum advantage. I happen to have a "strong Ambush contingent" in a couple of my WotR army and, though I would love that for myself, I don't think it's a fair way to play and am glad to see others feel the same. Now if GW came out with an official ruling to the contrary I would be sure to leverage it.