I'm not clear on what "for cause" means for an acting director of the FBI. Wasn't the justification for firing Comey that he mishandled the Clinton email investigation? It looks blown out of proportion to me, but that might count as cause.
There are no statutory conditions on the President’s
authority to remove the FBI Director.
From the same cite, a President can nominate an acting Director. On the day after the death of long-time Director J. Edgar Hoover, L. Patrick Gray was appointed acting Director.
The Constitution was set up with checks and balances. But it requires the branches of government to do their duty. When you have a corrupt President of the same party that controls Congress, that may be asking too much.
I believe the parent is incorrect in that if he becomes the head, he can be fired for-cause. I also believe this is only if it he is appointed head, not if he is just acting-head.
[1] http://www.latimes.com/nation/na-la-fbi-deputy-director-2016...