Usually the medical examiner, coroner or county, depending on local law, which arranges burial or cremation and keeps records of the disposition.
The medical examiner case file, burial or disposition permit, county indigent burial logs, cemetery plot maps and records of retained samples.
Report it to the investigating agency or medical examiner with your evidence, rather than publicizing a possible match yourself.
Not informally. Exhumation must be authorized under applicable law, typically involving the medical examiner, courts or public health authorities.
Older practices varied, samples were not always kept, and some burial locations were poorly recorded, so ask what exists and record each answer.