Good luck with that claim. She pretty much did a Cook’s Tour of the amplifier showing the things that weren’t good quality and the components that were below spec. She has nothing to worry about on the issue of "libel," not least because her attention was drawn to the device when she was consulted on a remedy for a major failure of one unit which she identified the defect in design and created a solution to fix it, which then Willsenton implemented ("please tell the jury why, if it was OK already, as you say it was".) Willsenton then also improved the quality of the resistors and capacitors, some of which were under spec, and a choke, also under spec. She compared it to a Muzishare amp at the same price point, which is totally fair game for consumer review.
She isn’t offering engineering consulting services, so the point about professional credentials is moot. She is a skilled hobbyist AFAIK.
YT isn’t the only web venue where R8 modders are active, they are on AK, Reddit and probably other places I have not seen. I have seen other mods different from hers on YT as well.