This is a good discussion. The opening question, "Are you risking your spine Mountain Biking" is so generic. I guess the short answer is yes, but knowing your personal limits can help reduce the risk to personally acceptable levels. Are others still going to bellyache that you are living dangerously and not considering others (family etc.)? Yup!
To find those limits do you need to go over the invisible line that results in injury? It seems likely. Who do you know that rides a lot and hasn't ever been injured?
A coworker once told me a wise man learns from his mistakes but a brilliant man learns from the mistakes of others. That was about 15 years ago. He was a well accomplished ex pro roadie who then worked in the bike industry. He now does ironman's in his late 50's. A good egg and a wise man.