They must be really stuck in there. I would not boil the nipple portion. Maybe try a heat gun targeting the nipple and keep putting on some Breakfree " lubricant to soak into the threads. That worked for me on a couple of old muzzle loader nipples that were rusted in place . I would try that 2-3 times before grabbing them with vise grip pliers .
It wouldn't hurt if you could find a container to soak the nipple portion in after heating up the nipples with the heat gun. It takes time to soak into those threads.