For my Mamiya 645 Super and 645 Pro and my leaf shutter lens, I set the camera shutter to 1/8, the lens shutter to whatever speed is appropriate, and the stop down metering switch on the lens (the "A-M" switch) to A.
If you don't want to use the leaf shutter for a particular shot, set the shutter speed indicator on the lens to "F", and the shutter speed indicator on the camera to whatever speed is appropriate.
The regular shutter release on the camera is the one you use - you just have to remember to cock the shutter on the lens each time.
On my cameras, the camera's battery powers the meter in the metering prism. If I recall correctly, I believe it is the same for yours.
You are correct about the X synch on the lens - as long as you are using the leaf shutter, that is where you attach the flash.
I'm assuming you have the manual. If not, here is the link to the pdf on Mamiya's site:
Congratulations - enjoy yourself!