Try a shot of CRC "FOOD SAFE" penetrating oil on the outside of the valve holder. It will seep down to the bottom of the threads. Then let it sit for a few hours/days.
The penetrant isn't magic, I still had to use a fair bit of force to break the valve free. But I was able to break it free.
CAUTION, be careful. Do NOT round off the notch on the valve, or you will have to grind the notch straight vertical again.