1) Is mana storage a thing? Ie can we spend some mana to infuse a crystal / charge a runic array, etc, and then use the stored mana later?
Once it's been absorbed by a living being, no. When it's still suffusing an area or in a naturally solid form, yes. Some people make their living off going to high-mana areas, charging objects designed to slowly
release stored mana, and selling said objects in areas with significantly less ambient mana.
2) Is ambient mana blocking a thing? Ie if ambient mana is like air, can mana pressure / vacuum chamvers be made using existent knowledge?
You don't know if magic has gotten to the point where it can completely prevent mana from entering or exiting an area. It can, however,
reduce the amount which actually gets through. Some of the stories you know of involved using mana-absorbing armor, accessories, or spells when entering the lairs of particularly nasty monsters;* without it, the hunters would've either died or... had whatever other nasty side-effects are born from exposure to too much mana at a time. You still don't know what such side effects even
are.
*While the monsters themselves didn't output mana, they
did make their homes in high-mana areas in order to maximize the amount they absorbed.
3) Dragons existing indicates that sapient non-humans exist in the setting. What are interracial relationships?
The other major intelligent races (elves, dwarves, and goblins?) are similar enough that some believe each race was broken off from one of the others through deliberate magical modification. Which race is the "original" is a point of major contention. Elves and humans frequently live in the same areas, while goblins and dwarves occupy a different set. You think it's rare to find all four living in the same place.
Basically, would Imperfect (if we mutate to look inhuman) cause us to face social issues on an institutional level? Like, risking being burned as an unholy offspring of cross-species union by a local church / facing exile from the country?
You might face discrimination from some other people of this world, but you don't know of any religions or governments which persecute modified individuals. Either way, you don't think most people would actually think you look good unless you're
very careful about where you absorb mana from.
4) Same questions concerning conjurer and haunted. Depending on social customs those two can be a boon or a curse.
Mediums have tried to distinguish their own trade from Necromancers despite them belonging to the same school of magic. Basically, calling up the souls of the deceased (for short periods of time) is generally fine. Binding animal ghosts is also usually fine. Going around and animating the physical corpses of dead creatures? Not so much. Animating the corpse of an
intelligent creature is a capital offense in some places.