Something doesn't add up.
*The a32 5G has higher power consumption than the a33 (both idle and load, the avg consumption of the a32 5G is already higher than the max consumption of the a33)
*The a33 has a faster processor ==> less time need to do the same calculations (more time in idle)
*The battery is should be the same
By all means the a33 should have longer battery life than the a32 5G

Reasons that this isn't the case:
*The a33 has more background apps running (But why don't they result in higher power consumption in the measurements?)
*The a33 has actually a smaller battery (but since the a33 is more or less in line with the a53 I don't think so unless that is also the case for the a53)
*The a33 turns off at a higher battery charge (Will protect the battery from aging faster.)

The screen flickering is probably induced not by PWM. The form of the signal under different loads remains sinusoid instead of the rectangle pulses of varying width. There is an admixture of the AC component in the LEDs DC supply (poor filtering after transformation from AC?).