1.Two 12v lead acids, yes.
2.Uses both at the same time.
3.The included 24v charger charges both at the same time.
4.3rd bat can be had from batteries plus, ebay, mail order, from the web. You're adding 50% more watt hours, so you *can* go further assuming you don't over use the extra speed that you also get.
5.Yes if you buy a 36v charger. Or you can use the 24v along with a small 12v automotive type charger, but thats a pain.