Can a unit declare a charge and leave a character behind? Ie to have the saurus block charge ahead but leave the skink priest so he doesn't die in melee combat.
No. Chars can charge out, but not the other way around. A character can only leave a unit by A: charging or B: in the Remaining moves sub-phase.
on page 101 it says that a unit can move in the remaining movement phase after a character has charged out of it. it says nothing about it being unable to march. do you have another source for the "no marching" assumption?
I can't find anything in the marching rules or the character leaving a unit rules to prevent the abandoned unit marching. Was this a limit in a previous edition?