Sirsa Padasana is indeed a challenging and deep backbend. I practice this pose about once a month and some months my feet come to the head and I can assume the full form. You’re right in encouraging your readers that before embarking on this to have a strong Dwi Pada Viparita Dandasana and Sirsasana I practice. I would add that one should feel comfortable in Vrischikasana I and II as well since this is essentially the same pose as those except on the head. Something you can do it partners to prepare for this pose is to have one person come to Sirsasana I and just bend their knees (without assuming a backbend). Then the partner can sit behind them and place one foot between the shoulderblades and both hands onto their feet. Then push the shoulderblades with the feet and gently pull the feet with the hands. The person in Sirsasana can then experience the melting of the heart and extension through the legs necessary for this pose. All the person in headstand has to do is then focus on moving their hips back (just like in scorpion) to keep creating space in the space to extend the pose. Good luck!