Prior to cutting small PE you could place tape on the unpainted side so that when you cut them off the sprue they don't go flying everywhere. I use an xacto blade to cut the parts from sprues. Consider scoring the back of the PE parts with coarse sandpaper or a file to assist bond. Use CA glue to bond to plastic and make sure there is no paint on the plastic. For seat belts I usually bend the parts by hand and trial fit several times before applying glue. IMO it's important to get a natural drape look. Sharper bends can be done with tweezers.
You can bend them around a pin or something the right diameter if what you are looking for. Despite what I said I tend to use tweezers alot, making larger bends by inching tweezers along the belt and making a series of small bends.
Regarding painting .. you may apply a matal primer before painting. It prevents the paint peeling off. The primer is offered either in a bottle with a thin brush at the cap or in a spray can.
For the lap belts, I always bend them around the unpainted seat. This way I can make final adjustments and then put the P.E. in a 3 part plastic tray until needed. I love this little tray
It is one of the ways, Chris. But instead of scratching I would suggest using the Maskol liquid. It can be applied with a toothpick or with a fine brush that is usually attached to the bottle cap. Later , depending on the Maskol type, either you may remove it with the toothpick or just by the wipping. In the way the plastic under the Maskol remians unpainted.
the pic source: the net
Here the effect after using of the Humbrol Maskol drops for masking of the Bf 109E engine bed and its pin slots and the pilot's chair at areas where it had to be glued to the wall in the cockpit..