Well, for what I have been seeing from rausmutt he seems to have a full comic series. It has a lot of lore, so you might want to check on his Patreon for his comics.
For the third question: According to what the VN says, shelter the building absorbs the mana that is created by them to power its systems. That’s why they get what is known as the Shelter sickness.
Other than that, there seems to be mana created by anyone, and it gets to stay in the atmosphere what I can guess is that, yes, mana seems to be more abundant on other places.