Can an RI DUI manslaughter charge be reduced?

In some cases, yes. Skilled defense investigation can sometimes lead to reduction to lesser charges such as driving so as to endanger resulting in serious bodily injury (§ 31-27-1.1), DUI without the manslaughter element, or other negotiated outcomes. Reduction typically requires substantial evidentiary weakness in the state's case - particularly on the causation element - combined with experienced negotiation.