I wish I could help. I am merely a forum poster like many others here. I would advise you to stick with whatever SBR are telling you rather than anything that I post. They have experience in these matters. However my advice would be to avoid being so confrontational or accusatory in your emails. I would reply to the email you posted with something along the lines of:
"Hi Cole, thank you for your email and your patience with this issue. I understand why Betonline would use 1 specific site as a reference point for all soccer stats. However, the Betonline rules are very clear on this point:
"Any prop bets such as number of corners, yellow cards or first goalscorer will be settled according to information on the official website of the relevant league or competition."
The official website for Serie A is
https://www.legaseriea.it/. Using any other website for bet settlement puts Betonline in breach of its own rules.
I would understand if Betonline had a rule saying it reserved the right to use another site at any time should the official site be down or unreliable, post incorrect information, or be subject to possible outside manipulation. Betonline would need such a rule to protect themselves. That said this wager has genuinely won, the official website confirms the outcome, and Betonline does not have such a safety-guard rule in place that permits you to use an unofficial site with the wrong information to over-rule the official site with the correct information. I respectfully suggest that Betonline's position in regards to this wager settlement is untenable. I ask that you reconsider the matter with these points in mind. Sincerely - zzzzzzz."
As a side thought the rule I copy/pasted was taken from the internet rather than directly from BOL's site. Also the rule as it is now might not be as it was in March when the wager was placed.