Mao's plan to "rule the world" via job climbing at MgRonald's seems funny; until you realize; there have been business tycoons who started from the ground up. Part-time employee, full-time employee, manager, corporate executive, management vice-president, management president, corporate executive officer, chairman of the Board. And this is just business; by the time he gets to that level, he'll have learned how Earth society, politics and military works. Maou is an exception to only Humans Advance Swiftly; he's Long Lived, he's patient. Give him a century or two to himself, and he really would be a rising Power on Earth. (Look at Vandal Savage!)
It makes even more sense after you get to know that he wasn't the overlord forever - he began as a simple trooper in the demonic army and managed to become the overlord the same way he plans to get back now.
The above idea is also the reason why Maou seems to easily adjust to a simple life, compared to most overlords, who would probably not last a month or two living an impoverished life. He's simply back to his roots, and has not forgotten how he rose to the top.
In Episode 7, Lucifer finds Emi's bottle of Holy Vitamin B and jokes that if she keeps drinking stuff like that, she might get a heatstroke like Ashiya. Except, he knew what the bottle actually contained and Alciel had too much of it, thanks to Suzuno. It was a hint that they already knew who Suzuno was.
And because Lucifer knew what it contained, how else could have Olba known about it?
In episode 6 Mao and co go to an old school building that is apparently a supernatural hotspot, thinking that it might have a link to Ente Isla. Chiho explains that it was closed because several students vanished there without a trace, as if spirited away. Lucifer explains that places where Gates have opened in the past are often subject to supernatural phenomena. If they're right and a Gate did open there, then the missing students were likely pulled to Ente Isla or some other parallel dimension and never found a way to come back.
School officials likely wouldn't close the building down if the missing students had turned up unharmed somewhere. In other words, a group of kids wound up in the middle of a Trapped in Another World adventure in Ente Isla, but they never came back and never got to see their loved ones on earth again.