ok, help me out. Are a lot of these notes and letters on individual frames reference so that those down the line(2ka or inbetweeners and beyond) can get the timing right and add whatever extra detail is needed? kind of shorthand to help make the KA's job easier?

The numbers correspond to individual layers, layer A, layer B, layer C, etc. They are individually labelled so that the inbetweeners, ADs, and coloring team don't lose track of the order of the frames in sequence. the lines are meant to communicate to inbetween artists how close they are supposed to be to each keyframe. So if the mark on the line is closer to the first key, the drawing will more resemble the first drawing.

He's referring to the various handwritten notes. These are meant for douga, giving specific instructions about how it should be inbetweened or clarifying some of the drawings and other elements like smears or blush.

Experienced animators tend to rely on that as they can clearly picture and explain what they want.