As predicted, progress is slow and steady.
The most satisfying way of working is attempting to solve as many unknowns in a particular area, and then moving on when the going gets tough. Going back to a set of routines with a fresh pair of eyes really helps.
Some areas that I've worked on:
1/ Car control (X Position, Acceleration, Gear Changes)
2/ Road splitting code (High level areas including the 'road splitting state')
3/ Elements relating to the current track segment
4/ Sprite rendering code (Very complex, so moved on temporarily from there)
I've successfully understood various memory locations, and slowly routines become understandable from the meaningless muddle they originally were.