It will only work if you modify it. I found instructions here http://betterfamilyphotos.blogspot.com/2013/01/yongnuo-rf-603-olympus-om-d.html If you're not comfortable with that cowboy studios makes one that works if you are trying to stay in the sub $100 category. Otherwise go for a pocket wizard.
I have used this with the Canon 7D with Max sync of 1/250s but still had a thin sliver of blackness so I dropped the speed down to 1/200s. So from my experience with it.... 1/200s. I don't think this trigger supports HSS mode. version 2 of this trigger should support sync speed up to 1/320s.
Yes it will. It will work on any camera system but the "C3" portion of its model number indicates that it will also come with the appropriate cable or it to work as a wireless shutter release also on the 40D.
If I understand your question correctly the answer is yes you can set different channels. So if the unit on the hotshoe of your camera is set to lets say 14 and the flash you want to pair with that camera has the intended receiver attached and set to 14 then it will fire that unit. At the same time you can have another pair set to another channel and fire it separately from another camera. As far as it working on a 5d? the lit I read said it would. I don't know for sure because I have not tried to use it on the 5d. But I think the C3 does. You may need to read and make sure.