Is it possible for my humerus to still break down further after having a plate and 5 screws put in 10 yrs ago?

No. The bone or the hardware can break but only if you have another significant fall. The bone usually breaks below or above the hardware.
Yes. You can get a fracture at the edge of the plate, because this is a stress riser. This is the dilemma - there is a risk of getting a fracture below the plate, but removing the plate entails the risks of another surgery, and for a brief period of time the risk of getting a new fracture through one of the screw holes.