Doesn’t solve your problem with launch(), but I found this on StackOverflow and it works on my system:
//https://stackoverflow.com/questions/47026297/use-processing-java-in-runtime-getruntime-exec
String sketchpath = "/Users/yourName/Documents/Processing/sketchFolder";
String cmd = "usr/local/bin/processing-java --sketch=" + sketchpath + " --run";
println(cmd);
try {
Runtime.getRuntime().exec(cmd);
}
catch(Exception e) {
println(e);
}