During DC restoration of a video signal how is the first sync pulse detected?
You're over-thinking this.

The Wikipedia article on Analog television has a lot more detail, but basically the sync pulses are the most negative (or most positive if the signal is inverted) voltage that appears at any time. Therefore, a simple peak-detector circuit is all that is needed to find them, and a "slicer" (comparator) separates them from anything else in the video stream by establishing a threshold that is slightly below the peak.

This dates from the early days of analog broadcast TV, where the AGC circuits depended on the sync pulses of the inverted video signal being at a constant (maximum) level, regardless of the actual video content.

Once you have sync, it is straightforward to sample within the front/back porch intervals to get the reference black level.