I still think it depends on what the impassable terrain is supposed to represent. I know a lot of people assume that certain pieces of impassable terrain are infinitely high for purposes of shooting and line of sight. Personally, I would still argue against it in the case of a mountain or a cliff. I don't think the rules are very clear in this regard. It is probably best to discuss it with your opponent and come to a mutual understanding before the game commences.
Exactly. Never let your opponent define terrain as infinitely tall, or "mountains", or anything more impressive than it really is. I have hardly ever seen a tabletop rock formation that was much taller than a warhammer fortress tower.
As long as you both agree, I don't see it as a problem. I've used "infinitely" tall impassable terrain in the past and it worked out fine. I have no problem assuming that a rock formation is taller than it is represented on the table. Realistically, terrain pieces can't be built too tall or they become difficult to play around, since you can't reach/see over them. As long as these things are discussed beforehand, I think it is fine to play it either way. Before I start to play, I always discuss with my opponent the specifics of the terrain pieces on the board.
Only if it has visual contact. Also, depends whether you want to just treat the wall as a garrisoned building, in which case it doesn't answer the question.
What did they do with their fortress wall terrain pieces (ruleswise) for eighth edition? The garrisoned buildings rules sort of assumed a one story house/tavern and I remember some dodgy bits about upper floors...in case of an isolated tower. That's all I remember,,,