Marching to the sound of guns is modified by several factors, explained in the following link in the wiki
http://www.ageod.net/aacwwiki/Marching_to_the_sound_of_the_guns
Some things may have changed in the MTSG mechanics over time, but this is my understanding. Posture is factored in, if the corps is in defensive posture it receives a -10% chance of MTSG. Putting the corps in passive posture means it won't MTSG, but that may have some dire unintended consequences. The general's strategic rating also has some bearing on whether the corps MTSG.
If the battle occurred on the 8th day, the corps might not have been able to get back to its starting position by the 15th day, unless the regions are linked by rail.
This is a little bit of an exploit, but I try to always have a small unit independent of the corps in the same region entrenching. That way if a corps MTSG, upon its return the trenches will still be manned and in place.
Though I don't think it unrealistic that trenches created by several thousand men over several weeks should still be in existence if the men leave the trenches for a week or two, rather than completely disappearing if they leave their trenches for a few days. This could be something to be worked on if there is an AACW2, maybe entrenchment levels degrade one level per turn rather than all at once.
Ditto if I plan an attack, I will leave some small force in place in case the attack fails and the attackers need to fall back to their trenches.