What is the lifespan of an automotive clear coat?
How about big dings and repairs? Can one make spot repairs and what would you use? Varnish?

I can help you with those questions. The life span of modern automotive clear coats is very long now. Most large manufacturers have been developing their clears for decades and have hard to beat gloss and superior UV filters. They have huge resources to develop product, and the aftermarket paints available to the automotive repair industry are actually superior to the products used on new cars, believe it or not.

Years ago automotive enamels required regular re-paints, but when acrylic enamel and the current acrylic urethanes were brought into the market re-paints became a thing of the past. When was the last time you saw a faded car paint?
The life of car paint is now measured in multiple years and decades.

Yes, it can be spot painted, but you would need a qualified painter to do it. You probably can use marine varnish with some preparation, but it won't have the same durability as the auto clear, and the gloss may look different.