Naval snipers...
That silhouette looks like flat tops.
The scene with military attache. How is the situation around the world during that era? So it’s like part of their embassy in all important countries they’re not in war to monitor the latest situation?
The later chapter even have Orcsen held a military college where other countries (besides Elfind of course) sent their military delegation to hear lectures from Orcsen about the latest military strategy and trends used in the war, all while the war is still ongoing. And the story treat it like it’s normal thing every country always do in their world.
Could be. Maybe the Japan of this world made their own Battleship-Carrier hybrid, a Battlecarrier of sorts. Could be a giant steel beam drawn too big or maybe there's an irl warship that looked like that.
No idea what the global situation as the manga is focused on the Kingdom of Orcsen, but everything point to this world being at the same level as our own late Victorian era. Orcsen is clearly at the center of progress so it makes sense all other powers have embassies there even though all of the non-human species are called Demons. Could be all sorts changes seeing as how France is Gloire and Britain is Camelot in this world. As for Military attaches, it is pretty much the same as spying, but legally allowed to by the host nation. I think Orcsen wants the world to know their strength for both the prestige and for deterrence, although the King does have plans for an eventual war against humans. Even if the other nations have better understanding, Orcsen gets to be steps ahead due to the king being isekai'd and having tons of knowledge far beyond the current era.
I've not read ahead so I could only speculate. Military academies, even today, do accept foreign students all the time. Lots of benefits to be had from gaining a friend from another country who could think like you and maybe eventually become a high-ranking officer in their country one day. Favors for favors and all that.
As for Orcsen being chill in the middle of a war, that's easy to imagine. It's like Germany invading Denmark. Elfind doesn't seem to be in a position to do a counter offensive. Orcsen outnumbers them, have superior technology, better tactics/strategy, and this chapter saw a large portion of their Navy get sunked. Militarly speaking, they'll lose, but Elfind could make things slow and annoying via diplomacy with other nations.