oocOrcs can see even at midday, they aren't vyssie, so if the troops had moved out they would have been seen, in which case the described attack would have been upon the ambush position. Therefore I'm going to ignore the last part of your post Eyvind.
Even had the Imperials plan come to fruition, winter was a bad time to try and get things alight, especially tent fabric which was damp. But in any case their plan was never put into practice. Some worgs were lost as they penetrated the defensive line, but the intelligent worgs quickly realised the nature of the danger and bypassed the remaining pits.
Soon they were into the camp, and then it was down to the Imperials as to how they reacted.