I have a couple of thoughts about where to place the life hammer or similar tools.
Maybe the transmission hump between the front seats of most cars would be a good place. It is in reach and it is a relatively safe location if the mounting lets go in a crash.
Another thought is about what I saw cab drivers doing with the angle handled tire irons years ago.
They used to keep one tucked beside the drivers seat on the floor beside the door. Again it was within easy reach for whatever you might need a tire iron for, in a hurry.
About seat belt cutters;
There are commercially available seat belt cutters that are light plastic with a utility blade molded into them. They are cheap and can be taped, clipped or sewn onto the seat belt, sun visor or another location that is easy to reach.
If you are in an area of the country that uses milk bags the small plastic milk bag cutters will work on seat belts too.
(Try it on an old seat belt at a scrap yard if you have doubts)
I like crash scissors for a lot of things, but I carry a pair of Olfa stainless scissors in my tool kit.
They are a bit pricey but they work really well
http://www.olfa.com/ScissorsList.aspx?C=53