The Demon World is actually the northern/eastern hemisphere of Earth.Some Demon cultures are based on China and Japan, plant rice and herd pigs, their congress also have a Mongolian name. It would make sense if they actually are on the same planet after all.
Several of the technologies and ideas the Demon Queen has originate from our worldThere's enough No Fourth Wall commentary usually regarding "dimensions" to suggest that she's quite aware of what happens our world. Using her Crystal Ball, she observes our world's history, politics, and economics and borrows the ideas that work.
Young Merchant and Grand Princess Fire Dragon marryWhen Hero Young Merchant brought Young Merchant to Demon World for the banquet it wasn't just to show him Gate City. It was also to introduce him to Grand Princess Fire Dragon. Demon Queen wants to encourage humans and demons to intermarry in order to break down the hostility between them. Young Merchant is an ambitious guy going into an untapped market (I.E. Demon world). Marrying a local princess would do wonders for his connections there. As for Grand Princess Fire Dragon, she seems to have a preference for strong men and Young Merchant has proven himself to be Hero's counterpart in business.
Demon Queen's original name was actually Ruby EyeIt wouldn't make much sense for her to be called Demon Queen before she actually became it.
Hero's concern of a Mayfly-December Romance is a non-issueHe's got every right to be concerned, but given all of his other impossible feats, it seems likely that somewhere in Hero's training, he's unwittingly become immortal, or at least extremely long lived and unlikely to feel the effects of age any time in the future.
People in this story do have names.I Know Your True Name is in full effect here, so everyone just uses nicknames and job titles to avoid this. The series will end with Yuusha and Mao telling each other their true names, as a sign of their absolute trust and devotion.
The Demon Queen is a Jesus analogueIn the preview of the 10th episode, a mysterious, omniscient woman made of rainbow-colored light with red eyes mourns the state of the world and how it's her fault. Given that the Light Spirit is credited in that episode, she is probably the Light Spirit. There are also strong hints in the narration that the Light Spirit is also the Demon Queen. The only stickler is how this works with the implication that this has all happened before with different heroes (and probably different Demon Kings/Queens). It could be a Gnostic thing like Xenogears.
The world that they lived in is the same as Spice and Wolf
Maou's original title before ascending the throne is Crimson ScholarGiven the way titles work in this story, it would not be that surprising. Furthermore, Maou could use that name/title in the human world with impunity since humans would not recognize it but any demon infiltrators would and keep clear of her.
Young Merchant's Dual Currencies scheme is motivated by a Roaring Rampage of RevengeWhile his scheme to devalue the Central Nation's currency through an artificial food shortage did result in profit for the Merchant's Alliance, it also rather viciously undercut the power of the Central Nations and the Church that pulled their strings. This could be in part due to the fact that they had declared his beloved Crimson Scholar a heretic and blasphemer for the sake of preserving their own power. The whole scheme was his way of bringing them to their knees in revenge. Notably this is the first time he acted without Maou's prompting to further her agenda and without a definite promise of profit. Also given how fast he managed to move in order to enact the scheme, its apparent he could have done this at any time.
The Side Characters are based on Real Life historical charactersGiven the schism in the church, it becomes startlingly clear that the setting is right at the beginning of the Reformation. That makes: