You perfectly encapsulated the disappointment we face when, after sifting and evaluating sources, there's no perversely satisfying conclusion. Your pursuit of the true tale of Uncle Harvey's end parallels the Don Murray hunt in so many ways. Both deaths carry the hypnotic scent of their respective times. We long to enshrine them as emblematic of their eras. Yet despite our yearning for a completed story, we (reluctantly) resist uncritically accepting unproven period themes and character embellishments.
As a fellow striver, I accumulated a stockpile of Murray related data, much of it peripheral to understanding exactly what caused his death.
Apart from engaging nuggets of info on Murray, his associates and family: I know every case where his autopsy surgeon testified, can draw the layout of the 1929 L.A. coroner's office, map the shortest and longest routes between the site where Murray was found, the emergency hospital where he was deposited, the streets to the next hospital and the mortuary that held his body. I know the history of these structures and institutions, their noted personnel and all relevant transportation schedules locally and between Chicago and Los Angeles in late May and early June.
I have a full dossier on brain afflictions, skull injuries, status of alcohol testing, police procedure and autopsy methodology as understood or practiced in the largest US cities in that era. I know the crime reports and attributed cause of death and cost to communities in every case that reached the coroner in Los Angeles (1928's national suicide capital) and the state of California over a multi-year span. I've studied well-documented cases - those with jury trials - from the 1920s and 1930s that might shed light on personalities and methods involved in or contiguous to the Murray case. Acquiring these materials was hardly a primary goal but we save what we find, on the chance it might lead to unlocking that door that refuses to budge.
Periodical accounts provided less than desired but, once untangled, more than anticipated. Still no answers.
One idea I played with didn't come from official or semi-official sources. It emerged from an anecdote (not about Murray's death) issued by a whimsical survivor whose recollections have historically run the gamut from truth to mad flights of fancy. Several of his checkable facts didn't hold up here. My guess is the "facts" were created as framework for a fragmentary impression he'd decided to enlarge. I'd discard his memory along with its collapsed cage except it makes a jagged kind of sense. On its own, insufficient. But it might yet connect to something I’ll uncover further on down the line.
I've run across other ideas that made a lesser impression, reflected contrivance, or deemed not worth tracking beyond my existing understanding.
PS - Your Gatsby quote was cause for rejoicing and is revered.