Serial controlled PWM

Hi. Im back to the circuit i started a while ago that uses PWM. Unfortunately the link in the other thread doesnt seem to work now. I have a few questions.

Do i need 2 interrupt pins? 1 to detect a zero crossing and 1 for the serial input?

1 circuit im making is for DC so it wont need the zero crossing pin but what about the serial input? Im sure it would take way too long using serin2 and a timeout.

On the AC circuit should the zero crossing be put through an optoisolator? Will a simple resistor be fine to lower the 9-18V input?

I could really do with making the AC circuit soon. I know i will have to use software PWM because i need quite a few outputs. Im just not sure about the interrupts because ive never used them before. I need to know that before i start designing this circuit and im hoping to have it completed within a week. I also have to buy the parts and etch the PCB so a quick response would be greatly apreciated.

Ive searched google but ive not found anything about using PWM and serial together.