It was also to do with how dates were stored in very old databases that were conceived in the 70s when storage was at a premium - a lot of them just used two digits for year.
Yes it sounds like they had to do some MacGyver level stuff at the last minute. As I understand it NDL (Navigation Doppler Lidar) was a NASA project bundled aboard the craft as a non-mission-critical demo but when the main naviagation failed they had to send a patch to the craft (about two hours before landing) to activate NDL and use it as the main navigation. Hoping more details come out about this.