Thanks for the comprehensive description!
If you put a 622 in the hotshoe, and a flash on the 622, can you use a 603 to act as a remote shutter release that can also fire the flash?
If you can, then is that the best solution? (since you said 622's can't act as a remote, and a 603 in the hotshoe can't fire the flash attached to it).
I tested for exactly that scenario and it doesn't work. It also doesn't work if you have a 603 on top of a remote 622. You need to have a 603 on the 622 on top of the camera for remote 603s to fire, which means a choice between firing your remote 603s, or having a flash on the 622 on camera.
What you could do is use a flash bracket to keep the flash about where it would be if it were on top of the camera (sort of), like this:
- Mount camera on flash bracket, 622 on top of camera, 603 on 622 and plug in shutter release cable from 603 to camera;
- Mount 622 on top of flash bracket, mount flash on that;
- Deploy 622 or 603 transceivers as needed. Trigger the whole setup (including focus/shutter) with a 603 in your hand. Half press of 603 focuses/meters, full press triggers.
Just bear in mind no TTL on the 603s. If you need to change the channel on them for any reason, the dip switches are in the battery compartment (I kept triggering another guy's stuff once while HDR bracketing the brooklyn bridge, so good to know if your stuff starts firing for no apparent reason).