What would be the most comfortable way to sit if you have lower back/lumbar pain?

Usually sitting is . The worst position as it generates 4x your body weight across the lower back while standing is only 2x your body weight & lying down is the least. The longer you stay in one position the higher the static load becomes like leaving a weight on a stack of newspapers for a few minutes v a hour-the stack compresses more. So sitting on a high stool mimicking standing or reclined in a recliner best!
Depends. Its important to maintain proper body/lumbar mechanics- some people like back support or a small cushion. It should be comfortable to you and not cause additional strain on your back.