1. If the Ents can walk inside Orthanc without any units, can they choose to attack the walls later from the inside? (ie. a Rohan army is waiting outside to finish off Saruman.
No, they must leave and attack from the outside. I'm not sure why you'd want to, though, since the Rohan Army can just walk right in and take Orthanc if there are no Isengard units present. If there are Isengard units present (mustered after the Ents entered), the Ents (being inside the Stronghold) can just attack them directly and eliminate them, along with Saruman.
2. If appears to be two moves from Orthanc to East Cun.. It appears one must 1st go thru West Cun. from Orthanc then to East, right.
"War is much more fun when you're winning!" - General Martok