Actualy almost 75% of the coding and programing needed to do that is in firefight for reach! As you can change the enemies luck, eyesight, hearing, how good their aiming is, they could use the campaign difficulty and make it apply only to one enemy unit instead of all of them. The programing is pretty much done. How else could they make firefight! Using the same tools they use to make firefight maps just without the map making dev tools.
So in the end it would only take about an extra week to make the extra tools and a month to convert all the current dev tools from firefight into a format compatible with forge. So it should only take them 37 days to add AI placement in Halo 4's forge. Remember if something has a good part and a bad part don't get rid of the whole thing and start from scratch, keep the good and get rid of the bad. Then you can use the good with something else where it has a better outcome. recycle your programing!