Did OEMs wet sand and buff paint?

You can still find synthetic/acrylic enamels. Without the lead and other nasty stuff the EPA made them remove I doubt they will hold up very well and will oxidize quickly if exposed to weather for very long. If the car is stored in a garage most of the time and you keep a good coat of wax on it you might be okay.

The old paints were nasty stuff health wise but the urethane based stuff is really bad. Some people are very sensitive to them and even a brief exposure will cause them serious health issues. You really need to take serious precautions when using them, all skin covered including your eyes. a fresh air breathing hood is best but you can use a good respirator if you use it properly.

Some single stage paints will let you mix additional clear in them some really won't. You'll have to check with the individual company's tech. rep. Some painters like to use a clear coat over single stage to give them a really durable finish. With a light color single stage adding additional clear may give you covering and hiding issues.