In (preposition of time) means during part of time. For (preposition of time) means used to show an amount of time. In a month means what something has happened or been done during a month. "I haven't seen you in a month" means that you have't seen someone for the duration of a month. The emphasis here is on when it happened. For a month means that something has been happening for the period of a month. "I haven't seen you for a month" means that you haven't seen someone for the period of a month. Here the emphasis is on how long it has been. Both of these sentences are correct it just depends on what you want to emphasize.

In the context you provide, both prepositions are fine, however, "in" is much more common.

Both of them can be used, but they have different tones. "for a month" is generally negative in tone. Something someone says if they had expected to see the other person sooner, or something someone my say in reply to someone who has business for them but has not seen them in a while. "In a month" can also be used that way, but it can also be a more neutral way of saying "I haven't seen you in a while." I'd stick with "in a month" unless you're absolutely sure you've heard it the other way in a similar situation.