Answered: use of 9 volt

According to <R13> The only allowable sources of electrical power for a VEX Robotics Competition Robot is any single (1) VEX 7.2V Robot Battery Pack of any type, unless the robot is utilizing the VEX Power Expander, and a single (1) 9V backup battery. Robots utilizing the VEX Power Expander can use a second (2) VEX 7.2V Robot Battery of any type.

Can a y splitter be used off of 9-volt to connect both to the cortex and power a Vex flashlight? Its purely decorative if that make a difference.

No, this is not legal. The 9V battery is not meant for this purpose.