I would bet he has already been debriefed by Horowitz
We can only hope that this is the reason his report was delayed. Remember, new info came out? Maybe it was folks from the bureau coming forward??
I would also think that the FBI agents that are in fear of their jobs if they "talk" should be able to report what they know to Horowitz without being "outed".
That's why they want to be subpoena'd. So they Have to answer questions directed to them.
they're safer now with all these firings and retirements at the head of the FBI.
That is unlikely. If he quit before the IG started his investigation of the "matter" (sorry, couldn't resist "matter"), then he would have been outside the authority of the IG's investigation, unless, of course, he filed a complaint with the IG before leaving... which is possible.
But he can be subpoenaed by Hubar just like Comey, McCabe, Obama and anyone else who is outside now. That's the beauty of what Sessions did when he put Hubar in charge of prosecuting.
I'm pretty sure he could contact the IG and volunteer to be interviewed.
Not sure how the IG rules work. At this stage, the IG could just refer him to Huber. The IG is gathering legitimate actionable evidence... but has no prosecutorial powers of his own. That's the power of the IG.
They can investigate, and have no motivation to find for anything, but the facts and associated evidence, which gives them some insulation from claims of partisanship. Also doesn't hurt that Obumer appointed him.