Some questions about the world-building (possible spoilers)

For 1 and 2, they didn't really go about it in-depth as to the how. They left it vague and as a "it's what we do kind of thing" for humanity. My guess is that in the current timeline they still have good enough technology since humanity was driven underground from the demon race. So while humanity was at the surface they had technology that was advanced. But after being driven underground they salvage what they could to maintain their militia. With it being vague you can say that the technology they salvaged is advanced enough to quell whatever criticisms you may imagine from comparing it to our current technology. It's left to whatever they could say. An example being that they have powerplants using magic that they learned from those they defeated in the past 400 years of war. It could give more credence with Jeanne having the mid-life crisis of questioning if the human effort of fighting the other races was for nothing. So those 400 years of war could just answer whatever questions you have of humanities technology. It's just up to whatever the people making the anime want to give us as a explanation. Still half baked answers until they give us more lore about it.

As far as logistics go after defeating Vanessa you could say they linked up with the other Human Hq for elves then the other one for spirits and it expanded their logistics and supply. Other than that we don't really know much more about Human logistics.

For 3, Reiren is the only one we see that has capabilities to smith elven bullets and enchant them. Jeanne uses an elven bow since it's more powerful than the standard weaponry of humanity at a cost of draining her life force. As far as why elves would still use bow and arrows, instead of guns. It's a cultural/technology thing. Why use something inferior in firepower for every person using it and is an insult to the history of elves and way of life. The Elve race isolate themselves from using technology from other races and focus on their magic. Kai's weapon is made from the tech derived from elven bullets after their whole race was wiped out in the different timeline. Humanity is the type of race to take chunks from other culture/technology in order to further their own race which led to Kai's weapon being created.