I figured out the problem.
The hotshoe on the Canon 6D was loose. It was causing the contacts on the yn622c-tx to not touch. Those contacts are a hair shorter than the Canon speedlite's contacts, which is why the 6D + speedlite was working fine.
I found a video on how to fix a loose hot shoe.
After tighten the 4 ...
When using a third party trigger with a different brand third party flash, the only way to know for sure is to test it or find someone else who has tested it and can confirm that what you want to do works. Even what should work in theory sometimes does not in practice when using third party components from different makers.
A YN-560 II could be a suitable flash for off-camera use, Strobist-style, but it's cheap for a reason: it's 3rd-party Chineses manufacture, and it's an older model. It's been superseded by the YN-560 III, the YN-560 IV, the YN-660, and YN-560Li models, and most of us prefer Godox these days to Yongnuo for expansion options. Unlike its younger brothers, the ...
It is a manual flash with one single pin, so yes it will work on camera.
But In my opinion, the best way to use a flash is off-camera, so you can still use it in many ways.
You could buy an external transmitter and receiver. There are some brands really cheap, or the same brand, like the RF-603.
If you later decide to get some with a built-in receiver, ...