I think the single power supply will work fine. Normal operation should be well below the max of your power supply.
You need to be going through all the setup documentation in the Wiki documentation labeled First Time Setup. In there is a section on motor and servo configuration.
https://ardupilot.org/rover/docs/rover-motor-and-servo-configuration.html
In that section it tells you that you have 2 options that will work for rovers like the mower. One is for separate steering and throttle vehicles where the left joystick on your RC transmitter will control throttle and the right one will be the control to turn left and right. It is also possible to modify this first approach to drive the mower off one joystick where left-right is steering and up-down is throttle forward and reverse.
The second approach for “Skid steering” vehicles and the left joystick controls the left drive wheel forward or reverse and the right joystick controls the right drive wheel forward or reverse.
All of these are personal choices and you see people doing different things. I elected to configure mine to separate the steering and throttle where the left joystick controls throttle (which also controls forward and reverse) and the right one controls to turning left and right. Now that I understand this better I will probably be changing mine to be on the same joystick someday just so I can drive with one hand.
If you don’t complete all the stuff in first time setup you will have trouble and be eventually circling back to get the stuff done so things will work.