I think they realized later on that their original plan would suffer MORE plothole than there are currently:
Spoiler for End of G senjou:
If we assume Kyousuke is indeed Maou, it would be extremely difficult for them to justify how Kyousuka as Maou could lead the terrorist attack that killed Haru's mother. There would be a glaring decrepency in term of age and all, and that could hardly be covered, even if "kyousuke's personality" would believe he is in his late teens early twenties.
Also, the other problem would be his stamina and sleep deprivation: Maou is quite active left and right and there is little to no chance for them to keep both Maou and Kyousuke active without using obvious "long inactivity" periods, especially considering how Maou is acting most of the time at night, so it would mean "kyousuke" as a whole have hardly more than 1 hour of sleep per day, which would just kill him, no matter how you look at it.
Finally, it also leads to issues regarding Kyousuke's whereabouts and activities that he would himself wonders "why the hell did I do that?". A lot of actions wouldn't pass without himself wondering what's going on, which never happened early on.
While the "current" explanation for Maou is way too convenient, it generates "less" plothole than what appears to be the original script.