Ive had success with xylene on copper, but stopped using it for health reasons, it's nasty stuff that will cross the blood/brain barrier. Be very careful with it. For me, not worth harming myself over a hobby.
I'd have to disagree, since there's no spots (?) on the reverse, and I wouldn't use water at all. Some of the spots actually look like they could be flecked off with a toothpick, for instance the one above R in Liberty on the shield. You won't find organic stains "as part of the metal."
OK, close enough. I would add, those spots look surface-bonded, not chemically-bonded, and best removed by physical means, with a solvent clean-up at the end.
My guess is that somebody has worked on that coin before trying to remove even more black. What we see now is what remains. That said, acetone or xylene won't hurt anything so there is nothing to lose by trying it. However, I do suspect that it will not be helpful. And no, I would not recommend that coin be dipped.
What I have done is, I contacted NGC through their site and NGC will get back to you, John Schuch II was the rep that worked with me. I sent him a picture of the coin and he will discuss it with you. Very good man and one to be a straight shooter. *Lisa Berzins and John Schuch II