It's not the hollow but your muscle control. Your muscles are struggling to control the boot so you get chatter and drift. Just keep practicing those T stops with the stopping foot behind the lead foot. Try just feathering the ice with the stopping foot and a perpendicular blade, this helps to set your muscle memory. As you get more comfortable with the feathering, increase your speed and the attack angle of the blade (edge leading, ankle trailing, your muscles will control the amount of stiffness you need in the ankle). Then start to put more weight into the stopping foot. Your weight is in the center of the blade. Also as you increase the weight into the stopping foot, try lifting the toe of the lead foot, this helps with your shift of weight balance back onto the stopping foot. Once you get this T stop, then you start to move the stopping foot out to the side of the lead foot. As you master this the stopping foot can then move forward until you can eventually stop with it leading. It's all about muscle control and weight distribution, especially the muscle control of the leading foot as you learn to do this.
As to the 2 foot stop and outside edge chatter, do the above and once you are able to stop with the stopping foot leading your outside edge chatter will have disappeared. 2 foot stops should be driven by the outside edge, not the inside edge. I loath teaching snow plow stops but its a necessary evil to get someone to stop. Once they have the basic idea of a snow plow I never work with that again, it's always outside edge drills like the one above. Not only does it teach you an outside edge but it also works hard on your one foot balance. It takes much longer to master a 2 foot stop this way but once they get there the technique is far superior to someone who started and continued working with just inside edge stopping drills. Once you get the outside edge, then work on your inside edge for stopping.
As to hollow, go with whatever you are comfortable with that suits your style of skating. I doubt anyone can tell you what hollow you should be on without seeing you skate. There is nothing wrong with 9/16" or 5/8", both will work equally well if that is what you are used to.