Nomura as we all know has been writing this story as he goes. This has caused many retcons. Heck DDD pretty much retconed BBS. As we have learned from the KH3 trailer, both Eraqus and YMX both knew about the Keyblade War and how it needed 7 lights and 13 darkness'. So why in the heck did MX even bother to think using only one pure darkness and one pure light would ever work and also why would Eraqus who also knew this be desperate enough to murder an innocent child to stop this? They both should've known it wouldn't work from the get go. It's an obvious plothole created by bad writing and just serves to make the characters look stupid and incompetent. (Well, more than they already are.) MX stating he was 'hasty' just doesn't cut it as a way to cover up this stupidity. Especially for a supposed 'tactical genius' that Nomura attempts to make MX appear to be. Nomura needs to work on his story continuity.