I think #2 is just plain wrong. The question, Have you seen soap? suggests the speaker is just looking for any soap, not a particular soap. The response, No, it should be on the sink suggests the topic is some particular soap. These sound more natural to me:
Have you seen the soap? No, it should be on the sink.
Have you seen soap? No, some should be on the sink.
I might even go a step farther and say the question is odd. I'd almost expect this:
Have you seen soap? Of course. Do you think I grew up in a cave?
Given the more likely situation that the questioner has dirty hands and just needs soap, he's more likely to ask:
Have you seen some soap?
It's also quite possible that this conversation is taking place between people who live together, say a husband and wife. One might quite feasibly ask:
Have you seen the soap?
This works because the other person knows the house and probably can tell from context which particular soap the speaker is asking about. Is the speaker in the master bathroom? It's probably known to each of the people in this conversation that there's a particular soap that's usually in the master bathroom. For whatever reason, it can't be found.
You would not say Have you seen the soap? in a grocery store, for example. (Unless this is a grocery store you know very well, in which case the same reasoning applies.) In a store, just soap or some soap is more appropriate.
#5 could be interpreted many ways.
Perhaps the speaker isn't quite sure about the name of some particular German beer. You know that flap of skin between your nostrils?
It could be intentionally condescending or dismissive, as if the beer is so bad the speaker doesn't want to acknowledge it has a proper name. I wouldn't use that gadget if you paid me to do it. The speaker can't believe people like that German beer.
The that could also indicate a particular beer to which the speaker is pointing or nodding. The meaning is lost in non-verbal communication.
The that could be part of they think that .... In this case it could just be omitted and the meaning would be the same: Many people think beer they brew in Germany is the best in the world.
In this case, the presence or absence of the changes the meaning, sometimes in context dependent or subtle ways. Compare:
Some people say that the contestants from Germany are favored to win tonight.
Some people say that contestants from Germany are favored to win tonight.
The first says there are some contestants that happen to be from Germany that are favored to win, not necessarily because they are German. The second says that merely being German is an advantage.
If we are talking about beer, honestly I don't think there's any significant difference made by using the or not using it. One reason is that there are so many German beers. If we say that the particular German beers that we can go out and buy today are the best, then it's a reasonable assumption that there's something about German brewing in general that is better.
Also, good German beer is an actual thing that people do talk about (whether they agree or not), and beers are frequently categorized by their country of origin. People generally agree that a beer doesn't just "happen to be" German. Rather, being German necessarily implies some things about the style and taste of the beer. Thus, the two senses distinguished by the contest example above are equivalent.
I can do this to the contest example, also. Say I tell you that it's a German spelling contest. Now being German isn't just some attribute unrelated to the contest: being German necessarily implies you've been around German your whole life. Now the two cases are equivalent, and it doesn't really mater if you say German contestants or the German contestants. Technically, these mean two different things, but a person will think, "of course the specific German contestants participating in the contest today are favored to win because they are German". Thus, in terms of what people think, that is, what you have communicated, it is essentially the same thing.