Half Battalion Deployed: If one selects this rule for a large battalion, does the unit stay this way for the whole game or can the commander redeploy his skirmisher bases back to the parent unit during the game?
I'm under the impression that no they cannot return to the parent unit during the game.
Engineers: Under the demolish rules would it be appropriate to use an engineering base to blow in gate or a impassable stone wall as well as a bridge?
I don't know and I don't have the rule book with me, but I'm sure someone else does.
Second to above, when does an obstacle become impassable. Clearly buildings are impassable, but when is a stone wall impassable? I am thinking of tall stone walls (>7' in height) like those surrounding the orchard at Hougoumont.
Terrain features are defined by however the players would like. There are some rules to follow, but even the rule book suggests that players have the freedom to adjust the terrain features to their liking, or for a specific scenario. So if you want impassable walls in your game, and everyone is in agreement, then add them. And if you want impassable walls in one of your scenarios, then just note that in the scenario details and there it is.