Number 1. Batteries should be in some type of enclosure, if not for vapors then for general protection. It does not need to be air tight, but well sealed and arranged to accumulate at the vent outlet. If you have sealed batteries, it just needs to protect the batteries and terminals from personal and other contact. A broom or other item could fall across the exposed terminals and ot will not stop arc welding until theres not much left. It could be transparent or wood metal, cob, whatever.
Ventilate flooded lead acid batteries to outdoors power or convection.
Roll out shelves are sometimes used to keep batteries accessable vertical and tight to a wall.
Lots of waterworks... be creative and create distance. Partitions, or a lightweight modular closet, removable for service. (think facilities. I always pay attention in large facilities power and utility rooms.) A jet of water could kill power electronics. Still a great location. You dont have to listen to relays and fans or they are drowned out by building mechanical!
#2 id send it all from the same place with the meter, disconnects, service entrance, mains distribution/critical loads distribution with dc distribution and battery and power electronics downstairs directly below the main panel. Having the battery in the location drawn I guess it would just be bringing a sizable circuit right from a fuse or breaker on the battery or battery bus, to the dc panel location that places it wher you need it and some sxtra ac wiring and communications cables. You should not have to get into the dc panel, but to maybe add circuits., for that reason, it could easily be housed in the inverter panelworks. Any more insight on the need to shut down power in a hurry helps.
And actually, first. Where does the utility power meet the house best? Or is there a location yet? I could assume the main panel location, but that trick does not always work.
#3 you should not have an issue with power. Sensitive signals might get over ride from ac lines in close proximity, but you shouldnt hear david bowie coming out of your smoothie mixer or anything.
If you use the stranded thhn housewire wjth a round profile, it will be twisted even. Part of the trick is keeping the two dc current carrying conductors the same distance apart as they travel electromagnetically shields the line, twisting helps keep this and probably ads some alternating spiral mojo to the flux, not sure on that one.