Must it be proved "beyond a reasonable doubt" WRT this type of case?
That is the standard for a criminal conviction. In civil cases, the standard is "preponderance of the evidence".
Most of the cases in this scandal have been civil (or settled out of court). Many of the potential criminal cases could not be brought because of the statute of limitations. This one was an exception to that because they decided the clock stopped when Shanley moved out of the state (an absurd conclusion, albeit with a good result!).