It's not safe to drive... PERIOD. If you had a SINGLE broken stud I'd say yes if you keep it under 15mph but missing two studs, not a chance if they are both on the same side. If you don't have two broken studs next to one another I'd say yes if you drove 5mph with an escort and both vehicles had emergency flashers on but you should probably just call a flat bed tow truck.
In my experience the main cause of broken studs is tire shops impacting them on too tight, essentially overtorquing the fastener. The G35 wheel lug torque is 80 ft/lbs which is VERY easy to over-torque but the 12mm stud is usually good for about 200lbs before the fastener is damaged (200lbs is easy to do with an air ratchet).
Another common reason for damage is people using slip on wheel spacers and not replacing the studs with EXTENDED studs to keep the same thread engagement.
Another common reason is people using mag nuts instead of cone nuts, remove a lug nut from another tire, is the bottom end flared like a cone or is it flat. The G35 REQUIRES the use of cone nuts and mag nuts will damage the wheel and not properly seat.
Another common reason is not torquing the wheel studs and driving, the "death wobble" puts shear load on the stud. Wheel studs are designed for clamping load ONLY and are not shear rated.
Another reason is potholes, checking sidewalks, checking the curb of the track, etc. This is usually immediate damage and you know exactly when/where/how it was caused.
The STOCK studs are 12 x 1.5 and 45mm long, it's a very common stud and any parts house should carry them.
While I strongly recommend removing the hub and pressing the studs in you can replace them leaving the hubs installed, just make sure that the new studs are absolutely flush and when everything is back together drive the vehicle around a few slow neighborhood corners, stop and retorque, hit a highway onramp/offramp and retorque again to make 100% sure the stud didn't seat a little more. DO NOT OVERTORQUE, if you're not using a torque wrench you're doing it wrong!
Btw, what are "steelies" ??