My guess is no one took switching back and forth from pro to amateur
bouts too seriously in Marciano's case because it was
known to be a widespread practice. No one gophers through anyone
else's amateur bouts, seeking some reason or other to discredit him.
Exactly. As I said before, many of Marciano's contemporaries, including Nat Fleischer and numerous sportswriters, clearly knew about his having had the Epperson fight and then gone back to the amateurs. If you look through the local newspaper accounts from that area within a couple years of these incidents, you'll see that everyone regarded Marciano as undefeated in the very area and time when this series of events had just taken place. People were not blind, deaf and stupid at that time. His pro record was a matter of common knowledge, and due to Wallace's high prestige at the time, the Wallace fight was very much known-about.
"First pro bout: February 21, 1947, kayoed Lee Epperson at Holyoke, Mass... Amateur bouts... lost to Coley Wallace in the All-East finals in New York in 1948"
-Waukesha Daily Freeman, September 16, 1955
Notice that both outings, with dates,
are listed by the same writer in the same article which was published in a newspaper and widely available to absolutely everyone who could read. You can find this information in all kinds
of contemporary accounts. It is exceedingly obvious that no one saw any contradiction here. Everyone knew and no one cared.
As you say, what Marciano did was technically against regulations, but it was hardly considered a heinous crime, no one was much concerned about it, and they most assuredly didn't subscribe to this kooky, backwards pseudo-logic that certain historical revisionists with not-so-subtle agendas are so fond of promoting. No one was covering anything up or being duped, there was no mass amnesia, the records were common knowledge, no one cared and no one questioned the legitimacy of Marciano's undefeated professional record, because that record was
legitimate, and it will remain legitimate no matter how much wishful thinking and paper-thin, fallacious logic is brought forth in biased, vain efforts to discredit it.