What is the correct back position on an MTB?
A straight back might avoid pain there, but it's going to create quite a lot somewhere else.

In your first rounded back/straight back comparison pictures you can see that the rider's pelvis is upright and the ischial tuberosities of the pelvis  - the 'sit bones' are on the saddle. In the second picture the rider's pelvis is rotated forward putting pressure on the soft tissue of the perineum. That picture makes me wince just looking at it. Chronic pressure on the perineum can lead to all kinds of complications, including numbness and erectile dysfunction (if you are a male).