Interesting that you're seeing the 8va bassa marks. I just checked the file and don't see them at all.
And, you're right: I missed the piano entrance at bar 22 entirely.
Yes, Pan sets the stereo spread. You can set it numerically, either in the staff setup controls at the left of the staff, through the Staff/Setup menu item or graphically through GraphOverNotes/L<->R icon.
The latter is especially useful for "wandering minstrels".
David