Hi,
I was in Jinghong in September, and I asked at both bus stations about all buses heading into Laos (I speak Chinese). The only bus they told me about was the daily bus to Luang Namtha. I've never heard of a direct Jinghong-Huai Xay bus, and I doubt it exists. Besides anything else, my bus from Jinghong to Luang Namtha was more than half empty: there simply aren't enough passengers for an extra bus.
HOWEVER, whenever the new bridge at Huai Xay is built, this may well change, and they may even start running direct buses from Jinghong to Thailand.
In October 2012 though, I'm fairly sure there were no Huai Xay-Jinghong buses operating. The Luang Namtha-Jinghong bus leaves every day, I think at about 10:30AM. It takes something like 7-8 hours, depending on how long it takes at the border. There are 3 or 4 buses a day from Huai Xay to Luang Namtha. If you want to catch the Huai Xay-Jinghong, you'll probably have to spend the night in Luang Namtha. Otherwise, you could probably make it from Huai Xay-Jinghong in a day using a series of intermediate buses (and you could almost certainly make it as far as Mengla), but it would be a bit inconvenient.
The only direct bus I've heard from Kunming is one which goes to Luang Prabang. I think it travels once a week, but I'm not certain of the timings. Jinghong-Kunming is about 8-9 hours on a very modern freeway.