You'll find all the necessary settings to tweak the duration of screen backlighting in the "Backlight" applet from either MS or HTC settings. It's ssometimes common with the "Battery" settings on some ROMs.
If you really can't find it, then I'll post the related registry keys.
Now, about the phone waking up during a Bluetooth relayed phone call, there are some registry keys to check and experiment, probably based on the light sensor :
You may check them and try to set them to 1, and see what's happening when the phone is in your pocket or belt pouch upon recieving a phone call with a Bluetooth handsfree kit.
This key, when set to 1, also triggers a screenlock during/after a phone call :
I personnaly set it to 0, as I was bored the phone locked after each phone call, even if it wasn't before, thus forcing to unlock between each call, when I had to call sveral people, or redial after an error.
To force autolock when the phone goes to sleep mode, thus avoiding any accidental call, the only necessary key is this one :
You shouldn't need anything else, but the other ones (especially the first two ones in the lighting management area) could be worh a try for your needs.