Using a 64 bit kernel for a 64 bit capable processor will guarantee that the kernel will be using optimizations for your processor. Not "legacy" or compatibility features for older processor architectures.

Using a 32 bit kernel will not help with any of your issues and I would say that its extremely uncommon to have problems because of using 64 bit kernels instead of 32 bit ones.

You are having backlight issues but I cant tell what hardware you are using.
According to google, its Nvidia, if so try switching from the Nouveau driver to the Nvidia driver which can be done easily in the "additional drivers" program.

If this doesn't help try installing:

mpb nvidia backlight driver

for your nvidia card.