RC hack: how to detect 5 leds that are ON at the same time?

If you're just reading the LEDs as analogue voltage levels you don't need any transistors. Just connect them up to your analogue inputs and read the voltage. You could even write a little investigative sketch to see what exact voltages you get. Your final sketch would just be looking at the voltage to determine if the LED is alight or not.

As for the buttons, I'd just go with a quad bilateral switch. This way you don't even need to know what those switches are actually doing, just put one of your digital ones in parallel and control it with an output from your arduino.