IG report states that the only things that were corroborated in the dossier were things he lifted from the internet. Further states that his source said it was all bullshit he picked up over beers. Raw intelligence? What a joke.
There is no confirmation about what he did at MI6 but IG report states that his position was not senior. The Head of the Russia Desk thing is laughable bullshit lifted from a glowing early article about him, and then quoted by every lazy reporter since. NFW was he in charge of Russia for MI-6.
The dossier was the driving force behind seeking a FISA warrant. It’s right there in the report. You should read it.