Hello everyone,

My living room has (had) beautiful natural red oak hardwoods installed circa 1961 which were coated in oil based polyurethane (masterline satin). I am in Georgia. I have bedrooms, a hallway, and a living room all natural red oak.
My situation is very complex with far too many cooks in the kitchen I will try to be as succinct as possible but the story is nothing short of a nightmare.

My current problem is that I have unsightly chattermarks in my living room purportedly due to an unlevel floor due to some floor joist sag (visible dip on one side of the room that has become more apparent with subsequent refinishes) potentially also due to the floor being on 24" joists, and loose subfloor (R/O harmonic vibration from incessant sanding). According to my latest floor man all of this is causing his 350 lb hummel beltsander to bounce and cause chatter. Moreover, that the loose subfloor and wide spaced joisting are the bigger issues making the floor bouncy with a heavy piece of equipment. I am wondering if employing a multi disc sander or hard plate system might be means of ameliorating the chatter and saving my floor.

I have gone under the house and there is a joist which is sagging which should have had a pillar/support during initial construction - this would account for the sag on some of the left side of the room but it is also possible that the first refinish attempted to mitigate some of the unlevelness via sanding as well. After the fourth and fifth refinishes, the sag has become way more noticeable. I am going to brace this area with a jack/pillar etc but I am not going to jack it up at this point to raise the joist to where it should be for fear of creating other problems. If I have to replace the floor I will completely level the new floor surface by reinforcing the existing joists, new 2x10"s inbetween the 24" joists, and scabbing and shimming under new subfloor to make the new surface level.

The floor has been refinished 5 times in total largely because of the people doing the refinishing were inept, the first of which by the least qualified - 4-5 years prior. After that initial refinish in 2018 I received an excellent refinishing job which was uniform and immaculate and was the pride and joy of my home.

Approximately 10 months ago, a contractor scratched the living room floor dragging a heavy item into the room. DIY efforts to locally mitigate (polish, mask) the superficial damage were unsuccessful and naturally made things worse. I later attempted to use bona acrylic floor polish and subsequently bona floor polish remover to remove the polish both of which were kryptonite to a small area resulting in a fisheye/rejection issue in a small area where the polish/remover seeped into the cracks and periphery of boards. I tried to recoat the floor four (4)different times and the fisheye never went away. In that time I had people using too abrasive buffing pads leaving swirls on the floor and orbital sanders that made it look like Freddie Krueger went ice skating in the living room. I also had people not stirring the satin polyurethane resulting in a nasty inconsistent semigloss sheen that was several times not leveled - puddled and curdled.
Then in consult with another floor finisher I elected to refinish as the only way to resolve the fisheye/rejection and clean up the coating mess. Parallel sanding did not remove the fisheye and created the dreadful chattermarks. Then I had someone else come in and sand at a 45 degree angle insisting it would remove the chatter and the fisheye in one swift stroke. It did remove the chatter but the fisheye was still there. Those bona acrylic polish products are the devil. Also they did not blend their edging work - there were deep swirlmarks and parallel lines to the wall. So the next guy comes in and weaves in new boards in the fisheye area - which should have been done two refinishes ago - he reported loose subfloor when he did it. And refinishes sands parallel - and the chattermarks are back, again!

The latest floor man could tell it was going to chatter while he was doing it. I told him I didn't want chatter and he proceeded in coating etc. I had him refinish sand at a 45 degree angle - I could tell that he was not confident they would come out and he did not give me a guarantee. I don't deny that my floor has problems but I will never know if his equipment or his abilities played a role in the final outcome. Finished product has residual chatter and more grit trash than I am comfortable with in addition to occassional buffer swirls. It's a Maco paintjob - didn't cost very much but isn't what I want. How one technician completely resolved the chatter via 45 and he could not is incredibly frustrating.

Nail headed are beginning to become visible on the far edge of the room near the dipped area. I think one more trip around the merry go round may be possible. Prior to tearing out this beautiful select & better/character red oak and starting over with a prefinished red oak product on a leveled floor I am looking for options. Please advise.

Yes chatter marks can be removed before the first coat of finish after sanding to bare wood. A buffer with a sanding head block will remove chatter.
It seems that your floor is pretty close to being ruined since the nails are starting to show. If they are finish nails the floor is pretty thin. Some floors are installed with box nails and the heads start to show after some sanding, too, but still have some life left.
The distance between the joists may have required thicker than normal flooring, so there would not be so much deflection in the beginning, which would be a plus.
Blocking between the floor joists before sanding from below would make a big difference in stiffness and help with reducing resonance type of chatter.
I would recommend that you beef up the floor joists and reduce low areas before any more machine work is done on the flooring. Blind nails that show before sanding the final grit can be set below the surface and filled with wood patch, then the floor sanded to remove surface filler that shows..
Once the floor is flat, if the sanding doesn't result in an acceptable surface, the new flooring may be installed over the existing flooring. It's recommended that new flooring cross the old finished flooring.

Unfortunately, I don't think it is possible to make the existing floor level there is a significant dip on the left side of the room and I am only willing to use a jack or install a column of blocks to prevent it from getting any worse.

I suppose the floor could be reinforced from underneath and then leveled from the top where the dip is prior to a new layer of 3/4" flooring

I have come across homes where the "weather" side of the house has sunk into the ground from expansive soil. Corners can sink because of poor drainage when a downspout empties near the foundation.
I have had some luck supporting a main beam with a jack and cutting the post where the hump is, then tapering other posts to allow the beam to settle over time.
Gradually the beam will settle down, usually without bothering the plaster. It takes a 20 ton hydraulic jack to take the weight off post to be able to cut them.