The main use of heaters in this setup is to make the "spring" of oscillatory heat system "stiffer". This should make temperature oscillations faster and smaller in amplitude thus bringing process to more stable state. This is mostly important when you have set temperature close to ambient outside, otherwise you'll inevitably leak a few watt through fridge insulation anyway, and if it's not enough you can always prop the door open. Latter would lead to humidity buildup though. If you set process to 20C, obviously, no leak, huge slow oscillations on heating phase.

Then there is diacetyl rest, I'd just recommend taking that thing out for that.