Sure, his report will hurt Cujo and give Kofi a black eye, but it won't tell the truth. It won't reveal all the crime, corruption and dishonesty that a decent investigator would uncover; that wasn't Volker's job. Volker was assigned to put the minimum amount of blame on Kofi et fil that he could get away with and that's what he'll do.
After the report, Annan will make some minor but flashy changes, admit no "personal guilt," and try to move on to "important issues."1 He needs a couple more years of thievery so he won't resign. Cujo already has other employment, probably with an organization whose future demends on his father's continued good will.
And Volker was complaining that he was being stonewalled, and having problems regarding interviews with people, paperwork, he even emphasised his own lack of power.
I think he wanted to (at the time) give a tip off to some senators, this report, regardless of what it says (and I honestly thought it would be a total vindication for Kofi) could be used as the matches for a senate inquiry, they can take this and whatever didn't make the report, and use them for bullets.
Senator Coleman has a career and a name to make, and this is going to be the perfect way to do it. Senator Coleman is already probing the UN, and it seems they are just waiting for this to go first, this will get the iron hot, and since the committee is set, they'll be able to strike.