Now of course this is a semi open case, but still, the inside of the case has hot GPU and CPU heat that come out. So only some cool air goes in, being used by the whole case, and part of that is being suck in by the PSU.
To answer this part.
There are no static areas of 'hot' air, as with 3 radiators on exhaust the air is constantly circulatling.
The whole 'cold' air coming in is a bit of a myth/misleading. You get ambient air coming in.
Air will always find a way into a case, air will always exhaust from a case - you cannot create a vacuum to control this.