The NTSB's function was previously handled by the Civil Aeronautics Board, which was also the parent agency over the FAA's predecessor agency, the Civil Aeronautics Administration (CAA). But, the NTSB wasn't within the CAA.
In 1958 both the FAA and the NTSB were created as independent agencies. A few years later the Federal Aviation Agency was folded into the Department of Transportation and became the Federal Aviation Administration.
I should have originally written, "... a major reason of why the NTSB was separated from the DOT".
The NTSB wasn't created in 1958, but later in 1967 when the FAA was renamed and folded into the DOT. But back then, the NTSB was also (administratively) part of the DOT, which created some conflicts of interests.
So in 1974, the NTSB was separated from the DOT and became fully independent. The role of investigators to find
probable causes of accidents and incidents -- instead of determining faults or to place blame -- is very central to NTSB's mandate as an independent safety board.
(As an aside, it seems that the Civil Aeronautics Board was never the parent agency of the Civil Aeronautics Administration either. Both were spun off from the Civil Aeronautics Authority.)
On possible administrative sanctions,
if warranted, even the FAA may have limited recourse in seeking enforcement actions against the AC759 pilots since presumably the pilots only hold Transport Canada licenses and not FAA certificates. The FAA may refer possible violations to Transport Canada via the State Department and the Canadian Embassy; However, it would be up to Transport Canada to take matters further (or not).