I've found it pretty much impossible to isolate Java web start versions. I have one program that fails if Java 1.8 is installed unless I:
- Edit the .jnlp file (which I had to save locally) to specify the version
- Launch all the following commands from the target JVM specifically
javaws -uninstall
javaws -clearcache
- wait 30 seconds
javaws -uninstall
javaws -clearcache
- wait 10 seconds
javaws (path to edited .jnlp file)