If you'd really like to sound more "natural", go ahead and replace "for" with "in" in those negative sentences.

Hi, they are both fine. The first one is more casual, the second one is more formal. So, in business situations you will use the second one.

I haven't seen you in ages or I haven't seen you in a long time. Both are correct and I would say both to someone I haven't seen in a long time. If you are looking for a native English speaker to talk more about situations like this I can help. I am from the USA and have over 4 years experience teaching adults and children of all levels. Chat soon. =)

They are both correct and natural.