I know enough about the legal system to know that it works like all other human institutions - maybe more so considering the stakes. Which means that much that it does cannot be justified.
Specifically, I conclude that that prosecution didn't use the trooper's report because it was seriously defective in some way. Not because the victim had no one to corroborate that she was at home at the time of the crime. Meaning that I believe it quite likely that she wasn't at home.
Without seeing the trooper's report, knowing more about the victim and her family, and talking to any surviving members of the D.A.s office at the time and seeing any relevent records, it's not possible to go further.
Presumably the Parole Board had done all of these things. Why do you assume that their conclusion is wrong?