Sure. Wash down with TSP, sand sheen off, Prime with Kilz primer. Apply 2 coats of paint. I prefer oil for durability but if going white, it will yellow over time.
They can be. But you will lose the protective properties. Cedar is a natural bug and mouse deterrent. When you paint it those properties are temoved.
Yes , you can paint them , but why ???? Besides cedar smelling so good , it will keep pests away ! You will ruin the natural pest fighting ability of the wood by painting it .
Seal the knots with shellac based primer so that it doesn't yellow over time.