How likely is it to develop mouth cancer from smokeless tobacco? 10% chance? Or less?

Greater then smoking. Chewing tobacco is worse then smoking. Chew is placed in the lip and sits in one spot. The nicotine acts as a constant irritatent to that area. The individual repeats this in the same spot. Smoking while they hold the smokie in there mouth it circulates around inuit the lungs and the out through the nose and mouth. Think about it if a irritant sits in one place for long periods of time.
No studies. No studies. Don't do it. See oral surgeon or ENT doc if you have any of these symptoms: Persistent mouth sore, persistent mouth pain, a lump or thickening in the cheek, a white/red patch on the gums, tongue, tonsil, or lining of the mouth, a sore throat, hoarseness or feeling that something is caught in the throat that does not go away, difficulty swallowing/chewing.
No one knows. A minimal amount so assume any puts you at risk for disease.