I'm trying to figure out why it's impossible for someone in Senator Fienstein's office to have leaked the letter.
Didn't say it was impossible, just extremely unlikely. Feinstein didn't want the letter made public, for the reasons listed, and Senator's staff members don't last long if they are not fully on board; typically staff & Senator feel like family to one another and are extremely loyal.
The fact that so many others knew about the letter after that confrontation, and disagreed with Feinstein holding on to it provides a long list of much more plausible sources, including from Dr. Ford's circle of friends.