As to this:
The fact that Feinstein sat on the letter for two months suggests that some of these factors may be in play,I think we conclude that Feinstein was holding the letter until the very end because she thought it was weak evidence to be used ONLY if nothing else worked.
She was probably holding the letter close to her chest because she was afraid that someone might share it with the opposition.
Apparently someone did anyway (ultimately resulting in the letter to Grassley from multiple women attesting to Kavanaugh's good character).
I would also suspect that Feinstein sat on it for two months to let Democrat operatives try to substantiate it. Apparently they’ve been unable to do so, so Feinstein has let fly with hearsay and innuendo.