Psu is underneath the bottom fans in a seperate compartment.
Airflow should be fine, fans are oriented correctly.
Trapped air won't be an issue, the rad is well above the pump, you won't get air pushed that far down once it's acclimated and up top.
Gpu has almost zero affect on a cpu at idle with a top mounted rad. What a gpu is capable of, and what it's actually doing are 2 seperate things.
As far as top vs front, with decent airflow there's actually very little difference overall. Front sees generally slightly higher gpu temps and lower cpu temps, top sees slightly higher cpu and lower gpu, when put in comparison to each other. Pull config front can change that as air is unrestricted to the gpu vrs diluted sp when the rad is in push.
What air you get is NOT the temp of the component. When a pushed cpu is front mounted and running 80°C, that's Not the temp of the air running out of the rad. What you get is air charged with upto 142w of heat, which is nowhere near the 1500w of a hairdryer on high. The air will only be slightly warmer than the temp of the coolant, which at most runs 40-50°C in a 22ish°C environment.
50°C air has very little impact on a cpu running 70°C+ or a gpu running closer to 80°C+.
What'll impact them both more is airflow, of which push rads are worse for the gpu, especially with decent exhaust capacity as the air doesn't reach the gpu to feed its fans.
None of that affects idle temps, which are more closely respondent to cpu power use.
I'm going to assume that you enabled Performance mode in bios for the motherboard, and most likely also enabled Performance power plan in Windows, which isn't going to allow idle states, it's just a jacked up power cpu doing nothing of value.