Do I think William was faithful to Kate during the first few years of their relationship? Probably not.
After they split and got back together? Probably. They were more mature and serious about the future.
And screenshot from Twatter below, showing RP's response to another tweet. I doubt the media would hold back if there was proof about PW, especially as he and Kate weren't particularly popular with them when the alleged affair is supposed to have happened. Wasn't it during the Hero Hazza/Workshy Will era?
I remember reading a report at the time that Kate and Rose apparently had a brief falling-out to do with their kids, due to an incident at a playdate. That would make sense to me as two of my neighbours are currently not speaking for that reason.
Hazza was allegedly looking at Norfolk properties himself around 2016, but there were also a few rumours circulating that he wasn't particularly popular with the Cholmondeleys. Something behaviour-related IIRC.
JMO but if there's a grain of truth in that, and Hazza felt he was being frozen out of the Norfolk social set that W & K were part of, he might have even more of an axe to grind. And find a way to blame the Cambridges for it. Also, Smeg would probably have loved to befriend Kate's close, discreet inner circle too - to leak gossip and to try and drive a wedge between Kate and her friends. And she couldn't get near Rose, and was bitter about it.