The first two questions are pointless. No one saying those things about 8 PM intends the precision implied in the questions. The meanings sought aren't implied by the word "until".
The two questions about Friday aren't pointless, but the meanings are ambiguous until more information is supplied.
Examples that resolve ambiguity:
"I'm working until ...
Your example isn't grammatical.
You could say
"What other countries are you in touch with people from?"
That's grammatical, but it sounds a little awkward, too.
In the context you describe, this would be completely natural:
"You said you talk to people from a lot of different countries except Brazil. Which countries?".
Your sentence beginning "Has or hasn't..." is grammatical, but it's unusual.
This is a more idiomatic way to include that alternative:
Has encapsulation made it easier to work with complex classes, or hasn't it?
It's not easy to search for such sentences. They're less common than leaving the "or not" alternative out, because the only meaning added is ...
There are some contexts, including this one, where both prepositions fit. They mean the same thing. For the past three days (and presumably nights) the person has not slept.
But there are other contexts in which they don't mean the same thing.
He was locked up for three days
means that for a period of three consecutive days at some point in the past, ...
"My hair is not much spikier than yours" concedes that my hair is spikier than yours, but claims it isn't much spikier.
That amounts to "My hair is only a little bit spikier than yours."
Preceding the word "much", the negation "not" applies to it. It doesn't apply to "spikier".
The difference between "gone to" and "been to" is that the former can refer to the departure only, whereas the latter suggests you went and came back.
For example, you might ask "where have you gone?" of someone you cannot see, but "where have you been?" of someone that has returned.
So, in the context you have given, where someone is asking themselves if ...