lots of people in this section have built WW bikes. Friction is by far the easiest to build and maintain. lots information scattered around in this area. Most of your question and advice are sprinkled here and there.
Now for the ones you asked. If you could find a way to use the WW clutch it would only spin when you gave it gas. I have never been able to use it. I never though that it was strong enough. Most of us just remove all the parts and put on a solid drive wheel.
For a clutch I like a lift mechanism. When you want to stop you push or pull something and the motor lifts off the tire. When you want to go again you throttle up and drop the engine back on the tire. Its a pretty efficient way to manage things.
Have fun and read read read. There are some good ideas around. If you have specific questions and cant find an answer just ask people here will be glad to help.