Diagnosis is currently very hard to do accurately, and can only be done by specialists and highly sophisticated laboratories. Most of the time, TLC is the most vital treatment. Proper care can only be prescribed by a qualified specialist. Often times, a mito patient will be started on a mito cocktail - which consists of vitamins and nutritional supplements. Sorry - at this point in time it is incurable.
Current evidence supports the fact that it is inherited.
The inheritance is complicated and beyond the scope of this document. It will be covered elsewhere on the website. Yes, it is progressive and degenerative - ultimately leading to death in a large percentage of cases. We don't know. Research into the disease is just starting, and it is a highly specialized and small field.