That seems very unlikely since you are saying both are talking to the LCD
but they are using two different i2c addresses.
Actually i did have the right address while testing it. But i removed it and copied here from another sketch.
That is why the i2cLCDguesser sketch gives you the full constructor.
I tried that guesser but when it says in serial monitor to "hit enter or click send", it does nothing.
About backlight, seems like the lcd.noBacklight and lcd.backlight is working. But if i want to dim it i need to use a PWM pin instead of the jumper there.