You will need the original remote to change inputs. If what's labeled as "1-touch play" in the Roku menu works to get the TV to switch to the Roku's input when you press a button on the Roku remote then you may be able to set up the TV to always power on to the game console. Once you're finished with the Roku, cycle the power to the TV using the Roku remote to have it power on to the game console. Note that this will only allow you to switch between the Roku and one other device connected to the TV.
Roku Voice Remotes provide very limited TV control functions, namely a Power toggle, audio volume up/down, and mute toggle. These are sent by infrared just like a normal TV remote, and so must be set to the correct set of infrared commands that will be recognized by your television.
The rest of the keys on Voice Remotes control the Roku, not the TV, and are sent via WiFi Direct.
There is also some very rudimentary CEC control enabled by what Roku calls 1-Touch Play. When Roku control keys are pressed on the Roku remote 1-Touch Play utilizes the HDMI connection to the TV to tell the TV to turn on if necessary and switch to the HDMI port to which the Roku is attached. Other CEC-enabled devices may also enable similar auto-switching functions.
There are no provisions in the Roku to directly control other auxiliary equipment you may have connected to other ports on the TV. The TV itself, however, may be able to relay power and volume commands to other CEC-enabled devices connected to it.
