Because my father had died 19 years before my assessment, and my mother was in slight decline during my assessment, my ASD clinic agreed to consider my detailed written lifetime behaviors, characteristics and comorbids.
I arrived at my assessment with an inch-thick pile of papers, photographs, previous diagnoses and, of course, my baby book in which my mother had unwittingly described evidence of a few of my earliest developmental disorders.
For those of a certain age, like me, it is probably the best we can do to collect evidence, and articulate our childhoods in detail. If a diagnostician won't consider such evidence in the pursuit of a late diagnosis, move on to other diagnosticians in the hope of finding one who will agree to an assessment without parental involvement.
My own experience shows that it can happen with a little nudging.