I'm just speaking of acetone, which is very volatile, explosive and while not lethal when inhaled in moderate amounts can make you dizzy or nauseated. It is simply prudent to use it in a well ventilated area, and keep it from open flames and sparks. Treat it like gasoline or paint thinner.
One thing you could try... This works amazingly well for bullion and jewelry. I don't see it effecting this coin in a negative way. Take a glass baking dish, line it with aluminum foil. Give the aluminum a little bit of a crinkle on the bottom, so that it is not perfectly flat. Sprinkle a pinch of salt and baking soda around the edges of the baking pan, leaving the center of it free of either. Boil some water in a pan (enough to completely cover the coin in the baking dish), when it comes to a boil add a couple of pinches of baking soda and a half pinch of salt. Let it boil for a moment so the baking soda and salt dissolve into the water. Lay the Peace Dollar in the center of the baking dish face down, the side that needs the most work should be facing up. Then pour the boiling water over the Peace Dollar. Let it soak for a couple of minutes without moving it. Then use some silicone tongues or other safe device to remove the coin without burning or harming the coin. This method uses a bit of electro chemistry to remove debris from the surface. It is safer than using dip. Just make sure you don't let the coin come in contact with granules of salt or baking soda. You can lay a toothpick in the bottom and let the coin lean on the toothpick, that will help prevent any scratches from possible salt or baking soda. You need part of the coin to make contact with the aluminum foil, or the electro chemistry will not work.
Here is an update on the coin so far it has seen distilled water soaks, acetone soaks, and hot water on a terry cloth. While quite improved it is still in need of restoration. I am in the process of trying MS70 on a few silver coins just to see what it does. Does anyone have any history using MS70? Any do's and don'ts?
What scares me about a dip on the reverse is that it might take too much luster with it. There is clearly a bit of luster left I just want it to stay in an AU grade and hopefully a strait one.
I'm not sure what I would do. The obverse has a nice original look to it. Even a quick dip is going to take that away.
It looks like you've tried everything to remove the crud and are left with the metal surfaces that have reacted to form toning. If you want to continue, I like @Mainebill suggestion, diluted EZest or MS70. Start at about 10 parts distilled water and 1 part dip. Start with quick dip, rinse thoroughly and see if if there is any change. Repeat as needed. You can dip longer or increase the concentration if it's taking too long The advantage of diluted dip is that it gives you extra time to evaluate what's happening. You may also want to practice on some toned junk silver to get some experience. If you have steady hand, you may be able to put a little on a small flat plate so only rev sits in the dip. Once again, diluting the dip will give you time to quickly remove and rinse if you don't have the right technique.
Everyone has their own recipe for a cleaning, but when I have one in that state, I use an ammonia and water soak..50/50 for a few moments, then dry and start the acetone soak. Never a problem for me using this method especially for a coin in that cond.
Don't forget, acetone won't touch on toning. It only removes dirt, grime, organic residue, and assorted schmutz. Once the metal surface reacts to forms sulfides, oxides, etc, acetone is worthless. Dips will remove the toning which removes some of the original metal (that's why luster is lost), acetone doesn't. It would probably be better to do the acetone soak first, to remove the organics and expose the toning underneath, then do the ammonia soak (although personally, I don't like using ammonia/water, but just my opinion).
After reading the directions on the MS70. It says to massage it into the surface with a q-tip. Can you just soak the coin? If so it seems to be a mild solution and one I would try first. The last thing I want to do is create hairlines.
Dipping the entire coin too long will kill the luster on the obverse, while it might not be enough to get rid of the tarnish on the reverse. If you apply the jewelry dip with a q-tip, it'll keep you from touching the obverse. However, you do NOT need to scrub the dip, just let it sit on the coin's surface for a few seconds and then wash under distilled water for a good bit, perhaps 30 seconds. Repeat until you get the desired results (or as close as possible).
MS70 is a base solution. It won't harm the surface of the metal. It's basically a very strong detergent. It will only help to remove debris stuck to the surface of the coin. There are times when it removes toning, but only when that toning was caused by a thin film of a substance sitting on the surface of the coin. Toned coins where a reaction or oxidation of the metal will not react to MS70. I understand you being worried about using Ez-est or some other dip. The longer the coin is exposed to it, the more damage that can occur to the surface of the coin. That's why I suggested the boiling water, baking soda, salt, foil and baking dish idea above. It creates a VERY mild electro chemical reaction that helps to reverse the oxidation on silver. I've done it with deeply tarnished silver, stuff that is almost black, and it's like watching a magic trick. The dark toning turns to bright silver. One thing you don't want to do keep your face over the dish while the process is working. The fumes smell like rotten eggs. Because really what is happening is the sulfur that caused the tarnish is being released and it's effects are reversed. You can watch a youtube video on it. Here is a Youtube video of it working. Apparently I forgot that you have to be touching the silver object for it to work. It's been over 10 years since I've done this. But I used boiling water or very warm water. The thing is, it won't hurt the coin or you. Unless of course you burn yourself with boiling water, so maybe just use really warm water. LOL Worst that can happen, is nothing happens. It is safer than dip though.
Before I use MS70 I soak the coin overnight in a glass jar with acetone. If the acetone is cloudy I will swish coin in another clean acetone bath. Remove coin from acetone bath. Lay flat on a folded clean white terry cloth towel and roll a q tip soaked with acetone across one side of coin, rinse, repeat on other side, rinse, repeat on the third side, rinse. Use clean q tips on each side. Then place coin in glass jar and cover coin with MS 70. Wait 3-5 minutes, then roll a q tip soaked with the MS 70 over the three sides again using clean q tips. Rinse coin under hot running water then a last quick rinse in clean acetone. I think you will see a nice improvement.
Update, I used the MS70 because I wanted to make sure that the surfaces still had luster, and what kind of luster before I tried a treatment that would remove any toning. IE: I just wanted to see the surfaces better. It is my understanding that toning is a part of the luster, and when you remove the toning you also take some luster with it. After practicing on a couple of junk silver coins, I went for it and gently rolled a qtip across the surface. After a pass with a qtip. I rinsed the coin off under hot tap water to remove any debris that I loosened. I did this same process until the qtip showed no sign of dark debris. It is now soaking in distilled water to remove any trace of MS70 on the surfaces. Unless someone sees something that I should deal with, I am gonna send this to one of 3 different coin albums to get a skin. Either A library of coins album. A Dansco A Wayte Raymond or, It goes into my coin cabinet. Any Votes?
If you really paid $27 for it, you can't go wrong no matter how it turns out. My suggestion would be to send it to the conservation service at NGC (NCS?). Surely their professional efforts would be preferable to the suggestions made on here. On the other hand, if you botch the conservation you can probably get your money out of it.