What is the evidence? No evidence W was unfaithful apart from a malicious tweet, quickly deleted.
Evidence:
The affair rumour was common knowledge amongst the aristo set for years, long before it got in the press or online.
William massively over-reacted (made a huge legal threat and claimed his "human rights had been violated") to an ostensibly innocuous story that didn't even mention him, about Kate falling out with a neighbour. Why would William feel so threatened by and be so desperate to shut down discussion about Kate not getting on with a neighbour, unless there was something deeper at play?
William using extreme legal threats to censor the British press from covering the rumour at all.
Kensington Palace issued a stern press release chiding Tatler and threatening legal action after they published a negative article about Kate, but in actual fact the only part of the article KP's lawyers had removed was a single paragraph about William and Rose, they didn't bother to have any of the nasty stuff about Kate removed. Why? Why was William so threatened by any mention of Rose he took legal action to censor the mere mention of her?
It was William's own behaviour in freaking out and banning any mention of Rose Hanbury that made people think there was no smoke without fire.
Rose Hanbury's brother gave a statement to the media to defend them against the affair rumour, and he acknowledged that William and Rose had private dinners just the two of them while Kate was away.
Multiple people who know royals personally have tweeted or spoken about the affair as being common knowledge.
Royal reporter Tom Sykes, who is from an aristocratic and royal-connected family (and who dislikes Meghan FYI) is on record as saying he heard about the affair from three separate people he regards as reliable sources, including the granddaughter of an earl.
It's clear that at the very least the affair rumour is all over the aristocrat set, which IMO torpedoes the blog post claiming Meghan started it. Why would aristos listen to and believe Meghan, a total stranger who's not one of them? The rumours started long before Harry and Meghan's wedding, do you really believe the first thing Meghan did on arriving in England was start spreading bizarre lies about her boyfriend's brother within his own friendship group? And they believed her?