I've got a hasselblad (200 series - close enough) so I may be biased
The camera vibration during firing is very low for a mirror that big. Sure it's a fairly imposing noise, but not much movement. Also, you can lock the mirror if you're really worried and there is no movement at all anywhere except the teeny-tiny shutter in the lens. The mass distribution may or may not help too; I'd be inclined to say it will help since it has a larger moment of inertia in pan and tilt directions. I have never actually used a mamiya 7 so I can't give a comparison, and I haven't printed that large so I don't want to quote a number. But it's less of a problem on the hasselblad than you think.

As for flash - I have a grip with a cold shoe on it and use a pc-shoe adaptor when I want flash. It's a pain in the ass and I really wish hasselblads came with a cold shoe somewhere. The lack of one is a monumental design cock-up in my opinion, and I simply don't understand how something so well designed and at the top of the market can have such a stuff-up. A cheap 3/8 screw in grip that I got for free (but would have cost less than $5) does the job just fine. Here is a fancy one that costs all of $19