I was curious about something related to the Navmesh Bug. If I edit an already existing interior, by adding new things to it: weapons, a second floor, etc. Will it suffer from the Navmesh bug?

Also I've seen a LOT of house mods popping up lately, so I've been thinking, maybe the Navmesh bug is fixed? Does anyone know?

Yes, that will be affected by the navmesh bug as well.

No, it hasn't been fixed yet. Some folks are moving ahead without it. House mods don't tend to show it off as readily because of their small size.

Even if I just prop some new weapons in the interior and/or mannequins and scratch adding the second floor?

As long as you don't need to edit the actual navmesh, that works.