1- At idle there is not enough coolant flow for reliable heater operation. There is a aux water pump in the cooling circut that is supposed to help with this problem but it does not seem to work all the time. I think you will find that the heater does not work too well when you are idling for an extended period. When you accelerate there is now plenty of coolant flow thru the heater core so you get a big blast of hot air until the climate control circut re-adjusts to the new flow rate.

2- I think the lighting for the ACC buttons are simply small 4w lights driven from the instrument panel illumination circuts. Are the dash lights varying in intensity? I don't think flickering lights in the ACC unit indicate a bad ACC unit - the lighting is independent of the internal components.

Tim