Since you said that you should consider peace, I have the courage to say things outside the law.
According to you, that ethnic group should have made trouble because they haven't found the ashes of their ancestors and don't know where they are buried. In my opinion, it is better to communicate with them calmly. First of all, their actions have violated the Law on Public Security Administration Punishment, which is certain. If it is serious, it can even constitute a crime of gathering people to disturb social order. They don't care about going through legal channels. Of course, this is not to let you confront them, but to let them know that this is your bottom line and you are obliged to make concessions again. When talking, invite village cadres or something, and everyone should be a witness.
Then, let them make an offer. Since they don't want money but ancestral graves, I have an indigenous way. I suggest you find a Feng Shui gentleman and follow their clues to find the location of the ancestral grave: since you care so much about your ancestors, there is no reason not to know the Feng Shui of the ancestral grave, right? If you can find it, it's best. You can arrange for Mr. Feng Shui to find another good place for them. It's not your fault if you can't find it. If not, we can only get a cenotaph. You must be a good feng shui master. In rural areas, such a semi-immortal figure plays a greater role than the law.
I hope the answer will help you.