First off one of your problems is you are trying to have the coin changer recognize 2 different prices, however the changer installed is only a single price coin changer so that will not work. You would need a 4 price changer but it is unkown whether the 4 price changer would work. Could it be done? Yes as I have made single price machines work with 4 price changers before in order to sell specialty coffee blends in single price machines.
The 50 cent change is a different issue and I am unsure why it would be doing that as the single price mech should still recognize the $5 credit and payback accordingly.