Am I correct that the spin axis of the ball is already tilted on sidehille lies? Therefore, if one zeroes everything out for a sidehill shot the ball will still curve (away from the slope) anyway?
No.
Only your path and clubface affect the axis.
What side hill lies cause to happen though is a change in our path and clubface... but nothing about the ball just sitting there on the hill affects it's spin axis.
I am not sure I understand this yet.
On a sidehill lie what does the top vector of the D plane look like relative to a target, say a pin. Is it a line from the top of the pin down through the top of the ball perpendicular to the earth or perpendicular to the sidehill slope?
Think about it this way, whenever the ball is above your feet it's like having a flat lie but you're using an iron with a very upright lie angle. Opposite when it's below your feet.
I am not sure I understand this yet.
On a sidehill lie what does the top vector of the D plane look like relative to a target, say a pin. Is it a line from the top of the pin down through the top of the ball perpendicular to the earth or perpendicular to the sidehill slope?