This is only a complete answer to your question from someone who both owns an FM and has verified the question both with the original manual and empirically, so of course it is only opinion :
- The Nikon FM meters through the lens (TTL). The FM will meter anything you put in front of the lens, including light transmitting filters.
- Additional exposure compensations, as stated, will underexpose your film if relying on the FM's on board meter. Your strategy will work, though I would prefer to make exposure adjustments with speed or aperture, if either using the camera and lens to meter a scene w/o filter, or using a separate light meter – though one may also meter through the correctly rotated polarizer off camera (a subject of much debate in itself), as we do with cameras without meters (e.g., LF)
- The FM shows the user all exposure information directly in the viewfinder, one reason, along with its all mechanical workings, this magnificent little camera was so popular. The aperture, in particular is mirrored from an overhang on the viewfinder prism housing up from the lens barrel to the top of the viewfinder.
- Polarizers in general will require an additional 1.5 to 2.5 stops, depending on the light in any composition. They are only useful in polarizing light when directed at right angles to the direction of the light, or in the case of reducing specular highlights. They may also be used to reduce exposure generally, in lieu of neutral density filters.