I understand why you want a diagnosis for your mom or dad; it would validate your experience. If you could get an official label for their behavior, an official reason for why they are the way they are, you could rest easy that you are justified in your struggles with them.
And yes, that might help. They might meet criteria and you may know the diagnosis because maybe they sought their own treatment and shared what their therapist or psychiatrist said. Or your extended family might acknowledge it, might agree with the way you see things. Or your own therapist might venture a guess (and that’s all it would be, really, an educated guess since therapists can’t diagnose people they haven’t formally assessed).
Most people aren’t going to get a diagnosis for their parents or else they one they get will be pure armchair. You read a self-help book about “Children of Parents with XYZ”