Old Fool...You nailed it...As I have mentioned many times on this forum there are true painters and there are applicators....anybody can apply paint, it's how it looks and lasts that makes the difference between the two. For the OP and I'm not trying to offend you, Single Stage paint was not designed to be clear coated...I don't care what is said about this, the fact is when it's sprayed and sprayed properly it's designed to be shiny. If it wasn't, it wouldn't be called single stage...I think they might give it a name like "base coat, clear coat". If it was designed to be clear coated why would it have a shine after you put it on your vehicle before you put the clear on.
The top surfaces are cracking in my opinion because the Single Stage was applied to heavy on the top surfaces and the more product you put on the longer it takes for the catalyst (hardner) to completely react with the paint (You can catalyze a gallon of paint and it may have a pot life of an hour or more, yet you spray the paint and it's sets up in minutes, not cures but sets up). I don't feel that it's a problem of trapped solvents as much as trapped catalyst. You have a catalyst for Single Stage...it may have a part number like LSH80 and your reduction is 4 to 1 for example, your clear coat has a hardner with a part number like DQ7500 with a reduction of 2 to 1...2 part numbers that don't resemble each other, 2 reduction ratios that in no way shape or form appear to be compatible. When the catalyst in the Single Stage hasn't had the time to react with the Single Stage paint and is top coated with a product with an entirely different catalyst who knows how long the product will last? I'm not a chemist and unless a person applying the paint is a chemist it's good advice to stick the tech sheets.
As I mentioned, I don't want to offend the OP, in the future if you paint a vehicle and you want to clear it use base coat clear coat. Your chances of having a problem like this occur will be minimized. If you did it this way because base coat cost to much....well, it took over a year to find your answer why base coat costs more.
By the way...I do seriously appreciate that you want to learn and I'm sorry that you had to learn the hard way...We all have had things happen like this....sometimes I think this trade is more of an art than a trade.