Mafia 67. Heart of the Void

1) Does the Mafia lose when only one Seer remains?

If only one Seer remains, and the Seer doesn't have a kill for the next night, the Mafia loses. If there is a kill, they win.
If only one Seer and one Sentiment remain, the Mafia wins.
If only one Seer, two Affections remain, and the Seer doesn't have a kill for the next night, the Mafia loses. If there is a kill, they win.
If only one Seer and one Affection remain, the Mafia wins.

2) Do the Maniacs know each other?

Maniacs can only win together, so I think it would be good to make it more difficult for them by giving them immunity to each other's investigations, or let them find each other during the game, or let them know each other from the beginning. What do you think?

3) How many investigations do the Mafia and Maniacs have?

The Mafia has one kill and two simple investigations.
The Maniacs, if they are together, have one kill and one simple investigation.

4) As I understand it, the Guard can only redirect when he uses his ultimate ability?

Yes.

5) Does the result of the Raven's investigation come to the Void when they have already decided on all the investigations?

Before they have decided. This is its purpose, to find out, for example, who Love will protect and not go to him.

6) How does the Archivist's investigation work with stat changes? Does he choose the new stats and for whom, or is it random?

The Mafia chooses for whom and what status.

And yet, I am in favor of slightly strengthening the town.

Perhaps, suggest specific options.