Ok, then the Leonardo will not allow as many buttons and lamps to be controlled.
Anyway, let's try the Leonardo for at least the motor control, pedals and buttons (no lamps yet).
For the wiring, you shall do the following:
- connect L_pwm pin to D9 (Forward)
- connect R_pwm pin to D10 (Reverse)
- connect L/R EN(able) pins to D11
- connect your wheel pot to A0, your pedals to A1 through A3
- connect your buttons to D0 to D8 and D12 (10 buttons)
If you can, please try with a lab power supply where you can set a current limit to something like 1 or 2 amps to avoid burning your BTS7960.
Do not switch on your power supply yet.
In the BFF (latest 0.5 version), use PWM_DIR hardware. Stop the manager and tick the "Dual mode PWM (L620X)" checkbox. Restart the manager and click on "Commit eeprom&reset". Please see below where those options are:

Calibrate your motor first, then your wheel axis (same procedure) and your pedals axes, then check your buttons are detected in the buttons page.
Then enable your power supply with limited current, and use the Diagnostic mode to check if motor control works.
Hopefully it will work with no modification in the Arduino code.
Edit: corrected a few typos and mistakes in my message