About the the types of injuries:
There has to be a balance between flexibility and resistance. Practicing aiming only for flexibility is bad for the joints, and also, very often people with too much flexibility and no resistance, have a less intelligent practice and it's more difficult for them do develop awareness.
For decades, Iyengar has been teaching Shoulderstand with the shoulders lifted on a support, and adjusting even more for people with any neck problems. Many non Iyengar teachers adopted the idea, but many haven't. Also, many many many students try Shouldersand (or headstand) long before they are prepared to do it. That is because many teachers teach arbitrarily, without any systematized teaching progression. Add to that the nature of humans of always trying to do more than they're prepared to do. I very often have studendts asking me "when will I start doing more advanced poses?"
Progressing in yoga is not just about doing more advanced poses, but about expanding the awareness in them, and cultivating a certain attitude of the mind. Otherwise, only gymnasts or extremely flexible people, or people who could practice 10 hours a day hidden in a mountain without any distraction would be able to progress!
Making your body fit a pose is something that requires physical action, intelligence of action, and an attitude of total involvement of the mind and senses in the process. The physical actions and the "design" of the pose are something to aim for, not only for physical health and balance -but because the way you position your body has a direct impact on the mind. Each pose has a different impact on the mind. So to assume the physical pose is needed. But assuming just the pose is not enough. What makes a practice spiritual is the total involvement of the mind and the intelligence in it.
In the Iyengar method, this attitude of total implication of the mind is developed through the simultaneous and constant observation of all the movements and technical details that compose the pose or the breathing exercise, while at the same time keeping the brain and its organs of expression (fundamentally eyes and ears) completely passive, and the control of the breathing. The intensity of this involvement of the mind is what makes the practice "spiritual" and a path to inner transformation.
Other methods have different approaches, but all the "serious" ones have in common this attitude of total implication of the mind. Of course yoga is not only poses and breathing exercises - Yoga is composed of eight limbs, yama (ethical standards), nyama (self discipline habits) asana (poses), pranayama (breathing exercises), pratyahara (withdrawal of the senses) dharana (concentration) dhyana (meditation) and samadhi (union with the divine, state of bliss). Iyengar systematized it so that in the practice of asana and pranayama the eight limbs are inherent in it. But that is why is is a method of yoga, not a "type" of yoga. Yoga is yoga.
So practicing asanas (with the right attitude) is a form of spiritual practice, and one that has a great advantage - you have your body as a support for it. Fixing your mind in an exterior object, or siiting down "meditating", trying to observe and control your toughts, etc, are practices that, in my opinion, are much more unlikely for you to succeed, and where you're much prone to be a victim of suggestion.
And about this devotion to the teacher... the teacher is just the medium. I trust my teacher about guiding me in my yoga practice, but I don't expect him to tell me how to live my life.
Janus, as an example I have to adjust my muscular actions everyday so that I can align my legs and arms and correct my hyperextended knees and elbows. I work my muscles to align my bones. That is what creates resistance in the right places and helps me avoid future injuries due to those hyperextensions. And I'm not really that flexible, not even close.
Of course yoga is a business - and most of the benefits associated with it are kind of distorted for commercial purposes...