This is pretty close to your theory 4 with some different aspects pointed out.
It’s better than theory #4, because it means no one could pressure lower level agents. I was not aware that Weiner’s case was on the NYPD beat.
Oh, Rolling Stone points out that the NYPD is involved with the Weiner investigation.
Then there would be evidence that the Weasel can’t get to.
That would explain it. The weak part WAS that the Weasel would let someone who wasn’t a crook investigate Weiner.
Having another agency involved, that explains it perfectly.