Weapon retaining bar, is a screwdriver good enough? To be honest I am planning to get a m5x150 bolt and weld a handle on it but having a idea of a backup is nice.
I've definitely seen meaty screwdrivers being used in the Beetleweight category to stop spinners. So long as it is as thick as a M5 bolt as you are planning, I can't see any reason why this wouldn't work.

What type of foam packing do people use round their batteries to help them not move?
I use Sainsbury shopping bags in mine plus the batteries are in a LiPo bag anyway. Works fine for my application.

haven't driven in anger just yet, but if the Center of mass is too far back, would adding a wheel bar type thing to force the nose down be a useful thing ??
Try driving it around in anger first but personally, I think you'll be OK as is.