OK guys I think I understood what you said 
I think I will just remove the trimmer anyway, since I don't need both PWM and manual control.
But I'm confused: do I really need a resistor if I limit the PWM output to 3.3v (as in my code) ? Knowing that:
- The 3.3V pin is rated 40mA max, same as the PWM outputs,
- When the trimmer is "fully open", there is no resistor between the 3.3V pin and LED-A pin.
So I think I don't even need to add a resistor!
Sorry, I'm still newb to electronics