Is Elmesia (?), the elf monarch, the head of state of the Sorcerous Kingdom? If so, shouldn't it be Queendom? Also, besides the really short fights with Razen and some of the religious zealots, as well a Elmesia's wariness, I still don't have a good idea of Diablo's capabilities nor the reason why he's so interested in Rimuru. I recall Diablo was very interested in the mask Rimuru's body's previous owner used, but I fail to see what that has to do with Rimuru since he wasn't around yet.
Elmesia's title is 天帝 or 皇帝 in Japanese, and both of these are gender-less words.
In any case, the English people probably had the same thought you did and defined Kingdom as being able to be headed by a queen also. The United Kingdom was one example until recently.
There is a reason Rimuru is special besides being from a different world, and is related to the mask, but that is the next story arc for the anime (LN vol 11) I think.