I sense there's a love triangle forming here, but I'm, not quite sure who the third member is.
get ready for a date would have been a highly amusing one. Alsiel plays the role of the house-husband to the letter, and though it's mostly played for humor there's definitely a significance there as it relates to their character arcs. I think it would be possible to take the whole date scenario a number of highly different ways. While there's no question Chiho has a crush on Sadao, it's interesting that he so matter-of-factly acknowledges it as a date. Does that reflect a commensurate interest on his part, of is it simply a reminder that despite his generally seamless acclimatization to the human world, there are still elements of it he really doesn't understand?
intruding on their meeting in the first place? Ashiya's reasons for tailing them seem transparent enough, by comparison. Is Emi simply worried about Chiho getting involved with Satan, as she claims, or is there something deeper here as anime convention would suggest?
at each other's throats in what for all the world sounds like a row between a girlfriend and a jilted ex-over, and the next they're shaking hands and Chiho is expressing her trust. Admittedly, there were certainly bigger things to worry about by then. We got our explanation for the earthquakes at least, via Chiho recounting her strange experience of a disembodied voice speaking to her in a language she'd never heard, but understood. Sadao tells us this is an "Idea Link" - a method of communicating across dimensions that's likely manifesting itself as earthquakes. And of course, all of these earthquakes seem to center around Chiho - including the massive one that strikes while the main foursome are squabbling at the kissaten.
left Sadao in his demonic form (which Emi quite considerately makes sure Chiho doesn't see). And it reveals the depths of Satan's evolution as a character - not only is he determined to rescue the humans trapped in the underground station, he notes that there were "surprisingly few casualties", a line which was delivered casually but which strikes me as highly significant for the reason that it was likely Satan's presence that limited the casualties in the first place. Satan seems to have be re-cast not just as a human but a humanist, and while we've heard plenty of perturbed navel-gazing from Emi about that we're heard surprisingly little self-reflection about it from Sadao himself. This is a comedy and a damn good one, but there are some very interesting and subtle story and character elements at play, too.