What is the last piece to be developed, and why? Statistically speaking, for white and black, what is the last piece moved from its starting position, and why?
I think a good approach to this is to see what pieces most likely move first.

Central pawns normally always come out.

The bishops and knights come out early

If you castle early, one of the rooks plus the king will have moved

The other rook is sometimes left stationary unless there is an open file.

The queen normally gets into play.

Since you want to keep the king safeguarded, especially if castled, I would say most players would try to move the pawns in front of the castled king last or leave them on their original squares.