Window AC Placement in an Open Living Space

I think you're asking where air would come from in a modern, well-sealed home, in order to replace that which escapes through an open window. The answer is simply that there's no such thing as a "sealed" home. Even brand new, ultra-efficient homes have countless tiny leaks. Door seals. HVAC penetrations. Recessed light fixtures. Gaps in vapor barriers. They happen.

Older homes were so leaky that it was almost as though a window was open at all times. Modern homes reduce that leakage dramatically, but you're still probably talking the equivalent of a 3-4" pipe through the wall. By opening a window, the convection and escape create low pressure that dramatically increases the airflow through those leaks, probably on the order of ten times as much.

There's also the natural mixing that occurs, which has probably as much effect or more as the overall leakage volume.

The way to go here is definitely the ductless mini splits. It sounds as though you have done your homework on this and you can always find what we call straight cooling units without heat. The mini splits are far more efficient than nearly any other system these days and you can choose which room you need to cool and how much it needs and when.You'll be glad you chose to go this way after its done.As far as stand by power, just unplug or turn off a breaker, but its not a lot of power loss either way. As far as over kill, you'll be glad if you add some overkill to an old home, they usually need it unless they are very well insulated and have great windows. Just speaking from experience.....good luck.