Your post reads like your contractor is going to keep tripping over himself as he tries one uneducated repair after another, with him possibly causing more damage to your floor with each additional stumble.

I recommend contacting the tech department that manufactured the sealer that he used. Find out what they recommend to strip the sealer. If you know for example that he used an Aquamix sealer, you can even go to their website and find a compatible stripper.

Since he "buffed" the sealed and dirty floor he might have buggered up the possibility of a simple repair, so refinishing of the entire floor might be a necessity.

Instead of letting him "sand" the floor, look into using a hogs hair pad and honing powders. Any time a floor is renewed/rehoned/polished, the entire floor needs to be treated in consistent fashion or you'll have variations in sheen in the treated floor. It's fairly easy to run a large 17" round pad over the bulk of a floor with a commercial machine. But getting the tight inside corners of the floor where the 17" disc won't fit in to match? That's where you can have issues.

You can let your contractor continue with his "repairs", but at least steer him towards the what I consider to be a more appropriate path. And ask how he plans on making the inside corners of the floor where the large machine can't reach look the same as the bulk of the floor.

Good luck!