Man, I thought it was bad enough when something similar happened to me in a 5k that turned into a 4k, give or take, last year (possibly my best race of the year but I'll never know because I wasn't expecting the finish line for another 3 minutes and I'll never know exactly how far I ran). This would be a nightmare for a half marathon.
The real problem is the different distances. How will they ever know who any rightful award winners were? Even in the race I ran, I have to wonder how the first person who was sent the right way felt. Maybe he could have caught some people in front of him had he been running the same distance as them. In my opinion, if the course is screwed up for the leaders, everyone should follow the same screwed up course. At least, if this is done, the results will have the correct finish order. Remembering that it's called a race, not a time trial, that should be the major concern, especially when it comes to determining who finished “in the money”.