I was wondering if anyone knew the canon reason that Cinder planned the breach.
There are 2 main reasons I could see, that might describe why this happened.
1) To cause more destruction and chaos during the Attack on Beacon and Vale. This is a reasonable assumption, and would make sense, seeing as they carried Mechs and lots of soldiers on the train.
2) To get Roman onto Ironwoods airship to control the Atlas Millitary. For this to work however, a couple things would need to happen, such as get Ironwood in charge of Vytal tournament security (which only happened in canon because of the breach), and to have Roman on the airship (which also only happened because of the breach).
I think the second option is the most viable to be Cinders plan, as it causes a large distrust of Atlas after the attack, which, as Qrow says, is Salems ultimate goal (to separate the humans). This leads me to believe that Cinder's original plan was to have the breach happen before the attack either way. This would mean that team RWBY did very little to alter Cinder's plan except to prevent much of the damage to Vale (which would have been superficial anyways).
Another fact that leads me to believe this is that Emerald states that the breach was only meant to happen a matter of days after team RWBY forced it to happen, while I believe that at least some amount of time, longer than a matter of days, the festival would begin. This is because there would need to be a fairly signifigant amount of time to transfer security of the festival from one party to another. There is also no indication that the festival was delayed for this, thus meaning that there was originally at least that amount of time left without interference from outside forces.
Well a Grimm attack right before the festival would create a demand for more security (provided by Atlas) that can be hacked. It would also lower morale and raise negative emotions (important for Grimm )
I think it served the purpose it was supposed to be used for - get Roman onto the Atlas airship so he was in the perfect place to take control of the Atlas robots, get Ironwood put in charge of security so that his robots would be the main force on the ground during the festival, and cause panic and strife through the city.
It was however set off days early, as you pointed out. I think she originally intended for it to be set off later, when the rest of the students at Beacon had actually left for their first missions. This would have reduced the amount of people in Vale able to respond to the situation, making it much more destructive than it was. Breach was overal not very destructive; it was contained to one area, and multiple teams were still in the city and able to cut off the flow of Grimm before they even got out of that area, and then the Atlas armada swept in and cleaned it all up in just a few minutes. A lot of people got hurt - but no one died.
Basically it sufficed for her plans, but it was anywhere near the amount of chaos she had hoped to spread and there were a lot of White Fang people caught in the blast, which led to real concerns over whether they would still follow her later plans for the festival. It was only because of Adam that she retained their services after so many of their people got caught in the train wreck.